Mr. Herbert Spencer--for it is to him we owe it--would be the first to admit the impartiality of his definition; and from the connection in which it occurs in his writings, it is obvious that religion was not even present to his mind. He is analyzing with minute care the relations between Environment and Life. He unfolds the principle according to which Life is high or low, long or short. He shows why organisms live and why they die. And finally he defines a condition of things in which an organism would never die--in which it would enjoy a perpetual and perfect Life. This to him is, of course, but a speculation. Life Eternal is a biological conceit. The conditions necessary to an Eternal Life do not exist in the natural world. So that the definition is altogether impartial and independent. A Perfect Life, to Science, is simply a thing which is theoretically possible--like a Perfect Vacuum.
Before giving, in so many words, the definition of Mr. Herbert Spencer, it will render it fully intelligible if we gradually lead up to it by a brief rehearsal of the few and simple biological facts on which it is based. In considering the subject of Death, we have formerly seen that there are degrees of Life. By this is meant that some lives have more and fuller correspondence with Environment than others. The amount of correspondence, again, is determined by the greater or less complexity of the organism. Thus a simple organism like the Amoeba is possessed of very few correspondences. It is a mere sac of transparent structureless jelly for which organization has done almost nothing, and hence it can only communicate with the smallest possible area of Environment. An insect, in virtue of its more complex structure, corresponds with a wider area. Nature has endowed it with special faculties for reaching out to the Environment on many sides; it has more life than the Amoeba. In other words, it is a higher animal. Man again, whose body is still further differentiated, or broken up into different correspondences, finds himself _en rapport_ with his surroundings to a further extent. And therefore he is higher still, more living still. And this law, that the degree of Life varies with the degree of correspondence, holds to the minutest detail throughout the entire range of living things. Life becomes fuller and fuller, richer and richer, more and more sensitive and responsive to an ever-widening Environment as we arise in the chain of being.
Now it will speedily appear that a distinct relation exists, and must exist, between complexity and longevity. Death being brought about by the failure of an organism to adjust itself to some change in the Environment, it follows that those organisms which are able to adjust themselves most readily and successfully will live the longest. They will continue time after time to effect the appropriate adjustment, and their power of doing so will be exactly proportionate to their complexity--that is, to the amount of Environment they can control with their correspondences. There are, for example, in the Environment of every animal certain things which are directly or indirectly dangerous to Life. If its equipment of correspondences is not complete enough to enable it to avoid these dangers in all possible circumstances, it must sooner or later succumb. The organism then with the most perfect set of correspondences, that is, the highest and most complex organism, has an obvious advantage over less complex forms. It can adjust itself more perfectly and frequently. But this is just the biological way of saying that it can live the longest. And hence the relation between complexity and longevity may be expressed thus--the most complex organisms are the longest lived.