And there may be good reason for the need of death among animals apart from the danger of over-population. The potential for variation of many species is important in relation to the part which that species will play in different areas of the world where foods, climatic conditions, altitude, temperature, etc., are different enough to require a modification of its form in order for it to meet the new exigencies of its habitat as it spreads or when the environment changes locally for geophysical reasons. We see this in response to temperature, for example, where limbs are shortened to conserve heat, where body shapes are elongated to dissipate heat, where fur covering density is adjusted, where protective coloring is modified, and where even home building patterns are altered as the supply of raw material is changed. But such adaptive responses occurring in any new generation have a much better chance of succeeding in future generations if the older generation dies off and thus clears the way for the new to take over entirely. The older generation would otherwise constantly be tending to return the divergent form to the old and less viable one, or the better adapted behaviour pattern to the former and less effective one.
Death thus becomes a benefit to the species.
Its advantage lies chiefly in its giving ampler opportunity for the genes of the newer generation to have their merits tested out. That is, by clearing the way for fresh starts and new combinations, it prevents the clogging of genetic progress by the older individuals. Having said in the first chapter that functioning protoplasm is not subject to death, it may seem contradictory now to speak of built-in or programmed limitations. The point is that such limitations are not inherent in functioning protoplasm, even as brakes are not inherent in a car engine. If such limitations did not exist, living things would continue on indefinitely and multiply accordingly.