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 rise 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.
To state and illustrate the proposition conversely may make the point still further clear. The less highly organized an animal is, the less will be its chance of remaining in lengthened correspondence with its Environment. At some time or other in its career circumstances are sure to occur to which the comparatively immobile organism finds itself structurally unable to respond. Thus a _Medusa_ tossed ashore by a wave, finds itself so out of correspondence with its new surroundings that its life must pay the forfeit. Had it been able by internal change to adapt itself to external change--to correspond sufficiently with the new environment, as for example to crawl, as an eel would have done, back into that environment with which it had completer correspondence--its life might have been spared. But had this happened it would continue to live henceforth only so long as it could continue in correspondence with all the circumstances in which it might find itself. Even if, however, it became complex enough to resist the ordinary and direct dangers of its environment, it might still be out of correspondence with others. A naturalist for instance, might take advantage of its want of correspondence with particular sights and sounds to capture it for his cabinet, or the sudden dropping of a yacht's anchor or the turn of a screw might cause its untimely death.