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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

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