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IMPORTANCE OF PRECOCITY IN EVOLUTION 321 



now all are practically toothless, but in the embryos of some it is credibly 

 asserted that rudiments of teeth " can be detected." The true Whale, 

 which when grown up has not a tooth in his head, the " Whale-bone " having 

 replaced an ancestral dentition, has manifest teeth while in the foetal condi- 

 tion. And teeth w^hich never cut the gums are present in the upper jaws of 

 our domestic calves before they are bom. How can these facts be inter- 

 preted except in the light of the theory of Precession, according to which the 

 teeth in all these cases, except where they are already extinct, are on the 

 way to complete extinction by constantly repeated precocity in their loss ? 

 Among the more civilised races of men the human teeth, to the alarm of 

 many who regard it as a sign of decadence, are now often lost early in life 

 — earlier, it is said, with us than in our fathers' and grandfathers' times. 

 May this not possibly be because the practice of softening the food by 

 cookery, most prevalent among the most civiUsed, whose offspring in all 

 probability will mainly inherit the earth, makes so powerful a dentition 

 ever less and less useful ? And may not our teeth have embarked on that 

 journey to annihilation, which has long since been complete among the birds ? 



For the exposition of further examples I have no space, but I would add 

 that none could be more instructive than the attempt to imagine the process 

 by which our modern birds and their wonderful powers of flight have been 

 developed from a reptile that could not fly at all. Whoever undertakes this 

 %\dth an open mind will, I think, hesitate to assign limits to the share that 

 constantly repeated precocity has had in the result. The astonishing 

 rapidity of the metamorphoses of insects ; the facts of hybridism as shown in 

 the ordinary mule ; and the origin of certain species differing little in 

 structure but much in habits, and on that account called Habitudinal, are 

 minor questions of Biology on which the theory of Precession will throw a 

 main of light. 



If, however, all characters alike tend by Precession to reduction and final 

 disappearance, how is it that any, even of the most useful, remain ? It is 

 because useful characters, such as the brain and the hand in man, the increased 

 efficiency of which now largely determines his selection and survival, are 

 still constantly undergoing changes and additions of structure, which involve 

 additional time spent in growth and a longer life : while the useless and indifferent 

 organs, one form of which is as good as another, gain no additions either in 

 structure or in time, and are thus left gradually behind. Hence the short life 

 of the tail in man, which is still conspicuous at a very early period of his 

 growth, but soon completely absorbed except a few still useful bones. This 

 is the way in which useless members and organs are discarded from the 

 organism. They go, not because of any considerable advantage gained by 

 those who have them in smaller size, but because every member and organ 

 which is to keep its place, and not be left behind by the rest, must undergo 

 additional variation, involving the consumption of additional time in its growth. 



To attain a clear view of the subject, we must consider it not statically 

 as usual, but as far as possible kinetically, as a matter of processes and 

 movements. Selecting the best terms to be found for its expression in this 

 light (poor as they are) we must describe the life of any animal, in growth 

 and decay, as a series of developments, positive and negative, the former 

 predominating at the beginning and the latter at the end of the series. Any 

 succession of animals in a line of descent (there ought to be a name for this, 

 the homologue in time of a species in space) is a succession of such series of 

 developments, generally similar, but differing in particular by the constant 

 addition of new developments at the end of each series, and the constant 

 suppression of old ones at the beginning. There is thus a clear contrast 

 between the methods of growth of Ontogeny and Phylogeny, but the accre- 

 tions of the latter are mainly made, not at the end of life, but at maturity, 

 the end of growth. 



