Different types of cells have different lifespans, e.g.: 

• we shed our skin cells about every 35 days

• red blood cells live about 120 days, platelets 6 days and white cells less than a day

• most of the adult skeleton is replaced about every 10 years

• the average age of a fat cell seems to be about 10 years

• a 25-year-old heart replaces about 1% of all its cardiomyocytes (heart muscle cells ) over the course of a year, while a 75-year-old heart replaces about half a percent

• our neocortical neurons, the cell type that mediates much of our cognition, are produced prenatally and retained for our entire lifespan

Many species of terrestrial plants, including the skunk cabbage that sprouts in February in the woods of Princeton, New Jersey, where I live, are warm-blooded to a limited extent. For about two weeks the skunk cabbage maintains a warm temperature by rapidly metabolizing starch stored inside the part of its anatomy known as the spadix, which contains the hidden flowers with their male and female structures. According to folklore, the spadix is warm enough to melt snow around it. The evolutionary advantage of warm-bloodedness to the plant is probably that it attracts small beetles or other insects that linger in the spadix and pollinate the flowers. The spadix is not a greenhouse, and the supply of starch is not sufficient to maintain a warm temperature year-round. No terrestrial plants are able to stay warm through an Arctic winter. On Earth polar bears can flourish in colder climates than trees can. It seems to be an accident of history that warm-blooded animals evolved on Earth to colonize cold climates, whereas warm-blooded plants did not. On Mars plants might have been pushed to yet more drastic adaptations.

Plants could grow greenhouses (so far the idea remains a theory) just as turtles grow shells and polar bears grow fur and polyps build coral reefs in tropical seas. These plants could keep warm by the light from a distant Sun and conserve the oxygen that they produce by photosynthesis. The greenhouse would consist of a thick skin providing thermal insulation, with small transparent windows to admit sunlight. Outside the skin would be an array of simple lenses, focusing sunlight through the windows into the interior. The windows would have to be small, to limit the loss of heat from outward radiation. The plant would also need deep roots, to tap water and nutrients from warmer layers underground. Inside the greenhouse the plant could grow leaves and flowers in an oxygen-containing habitat where aerobic microbes and animals might also live. Groups of greenhouses could grow together to form extended habitats for other species of plants and animals. An attendant community of microbes and fungi might help the plants to extract nutrients from the local ice or soil. Pores in the outer skin of the greenhouse might open to admit carbon dioxide from the atmosphere outside, with miniature airlocks and cold traps to keep losses of oxygen and water to a minimum.

Careful and reproducible observations and measurements in the bbiosciences have similarly forced us to repeatedly refine our traditional ideas about what life itself is and when it begins. Is a human being first a life when it emerges from the birth canal? Does it have any legal rights is as a person before then? Or is it a life at the stage of development where > it is able to survive independently outside of the womb even if it is removed from there early, as can happen naturally with premature birth or with a Caesarean section? But wait! Perhaps it is really a life when a fertilized egg first implants in the uterine lining, which, based on careful observations, is the medical definition of when a pregnancy begins. A woman cannot be said to be medically pregnant until her body begins the chem ical and biological changes that accompany a symbiotic hosting of the embryo, can she? If it does not, the egg, even if fertilized, is simply flushed. Now here we are getting into a tricky area, because many religious conservatives say, "No, it is a life when egg and sperm meet," whether or not the fertilized egg ever implants. But then, a scientist would ask, is it still a life at that moment, even if you know from careful observation that one-third to one-half of all fertilized eggs never implant?^ And of course that brings up a secondary point: What are fertilized eggs that never implant? How do we define them? As miscarriages? Abortions? Nonpregnancies? Something else? What implications might that definition have for the use of birth control pills that inhibit implantation? Is that abortion, murder, or pregnancy prevention?

It is an old saying, abundantly justified, that where sciences meet there growth occurs. It is true moreover to say that in scientific borderlands not only are facts gathered that [are] often new in kind, but it is in these regions that wholly new concepts arise. It is my own faith that just as the older biology from its faithful studies of external forms provided a new concept in the doctrine of evolution, so the new biology is yet fated to furnish entirely new fundamental concepts of science, at which physics and chemistry when concerned with the non-living alone could never arrive.

Muscles are in a most intimate and peculiar sense the organs of the will. They have built all the roads, cities and machines in the world, written all the books, spoken all the words, and, in fact done everything that man has accomplished with matter. Character might be a sense defined as a plexus of motor habits.

We know that nature invariably uses the same materials in its operations. Its ingeniousness is displayed only in the variation of form. Indeed, as if nature had voluntarily confined itself to using only a few basic units, we observe that it generally causes the same elements to reappear, in the same number, in the same circumstances, and in the same relationships to one another. If an organ happens to grow in an unusual manner, it exerts a considerable influence on adjacent parts, which as a result fail to reach their standard degree of development.

It will be noticed that the fundamental theorem proved above bears some remarkable resemblances to the second law of thermodynamics. Both are properties of populations, or aggregates, true irrespective of the nature of the units which compose them; both are statistical laws; each requires the constant increase of a measurable quantity, in the one case the entropy of a physical system and in the other the fitness, measured by m, of a biological population. As in the physical world we can conceive the theoretical systems in which dissipative forces are wholly absent, and in which the entropy consequently remains constant, so we can conceive, though we need not expect to find, biological populations in which the genetic variance is absolutely zero, and in which fitness does not increase. Professor Eddington has recently remarked that 'The law that entropy always increases—the second law of thermodynamics—holds, I think, the supreme position among the laws of nature'. It is not a little instructive that so similar a law should hold the supreme position among the biological sciences. While it is possible that both may ultimately be absorbed by some more general principle, for the present we should note that the laws as they stand present profound differences—-(1) The systems considered in thermodynamics are permanent; species on the contrary are liable to extinction, although biological improvement must be expected to occur up to the end of their existence. (2) Fitness, although measured by a uniform method, is qualitatively different for every different organism, whereas entropy, like temperature, is taken to have the same meaning for all physical systems. (3) Fitness may be increased or decreased by changes in the environment, without reacting quantitatively upon that environment. (4) Entropy changes are exceptional in the physical world in being irreversible, while irreversible evolutionary changes form no exception among biological phenomena. Finally, (5) entropy changes lead to a progressive disorganization of the physical world, at least from the human standpoint of the utilization of energy, while evolutionary changes are generally recognized as producing progressively higher organization in the organic world.

A vital phenomenon can only be regarded as explained if it has been proven that it appears as the result of the material components of living organisms interacting according to the laws which those same components follow in their interactions outside of living systems.

A man in twenty-four hours converts as much as seven ounces of carbon into carbonic acid; a milch cow will convert seventy ounces, and a horse seventy-nine ounces, solely by the act of respiration. That is, the horse in twenty-four hours burns seventy-nine ounces of charcoal, or carbon, in his organs of respiration to supply his natural warmth in that time ..., not in a free state, but in a state of combination.

What we call man is a mechanism made up of … uncrystallized matter … all the colloid matter of his mechanism is concentrated in a countless number of small cells. … [T]hese cells [are] dwelling places, communes, a walled town within which are many citizens. ... [T]hese are the units of life and when they pass out into space man as we think we know him is dead, a mere machine from which the crew have left,so to speak. ... [T]hese units are endowed with great intelligence. They have memories, they must be divided into countless thousands of groups, most are workers, there are directing groups. Some are chemists, they manufacture the most complicated chemicals that are secreted by the glands.