INTRODUCTORY. 7 



Nature has a long memory in that as she has built slowly through the ages, so 

 she now rebuilds in each individual development, repeating, over and over, in each 

 generation, the essential steps taken in the preceding generation. This drama of 

 recapitulation, in every stage a tableau vivanl, condensed and modified as of neces- 

 sity it must be, since every stage is to some extent a remolding of the earlier, 

 is nature's silent rehearsal of past history, or what is commonly called heredity. 



This is unquestionably the greatest wonder that biology has yet disclosed. Its 

 fundamental significance has scarcely yet found general appreciation even among 

 students of evolution. A few — Hering, Haeckel, Semon, Francis Darwin, and 

 especially Rignano — have grasped and treated its deeper meaning as a universal 

 mnemonic law, a law that underlies all intelligence as well as all development. 

 The disputations hitherto held over Haeckel's biogenetic law do not as a rule 

 touch the essential phenomenon; indeed, they obscure the marvelous accuracy of 

 hereditary recapitulation. 



This uniformity of nature's laws and the perpetual repetition of her building 

 processes are the two basal facts on which rests the hope of all cumulative science 

 and of our ultimate triumph in reading the secrets of evolutionary history. These 

 remarkable phenomena, which in normal development furnish, as a rule, only frag- 

 mentary and disconnected parts of past evolutional history, may be so expanded, 

 by suitable experiments, as to demonstrate complete continuity of stages in the 

 passage to the adult pattern. 



This crucial experiment consists in plucking a few juvenal feathers at such points 

 and time-intervals that the new feathers developing in the places of those removed 

 will appear at successively later ages, each unfolding a pattern in a stage of evolution 

 corresponding to its age. The result is that we get an ascending series of stages 

 rising gradually from the juvenile to the adult pattern, so that we have pictured 

 before our eyes the progressive transformation by which the earlier is converted 

 into the later and final pattern. One such experiment as this reveals a law that 

 must hold, in principle at least, for the pigeon phylum and all its branches; and 

 not only for this group of birds, but also for the entire bird world. 



Of course it is not to be expected that such an experiment would give equally 

 conclusive results in all cases, for different species have run different courses and 

 the length and fulness of the recapitulative phases vary greatly. But all develop- 

 ment, it must be remembered, is essentially a repeating or recapitulating process. 

 This is the central fact of heredity and the doctrine of descent. The first or germ 

 stage is, of course, the oldest in the phyletic series, and the adult stage, which we 

 commonly call the older stage, considering the age of the individual, is really the 

 youngest of the phylum. From this point of view it is readily seen that we are to 

 expect the fullest recapitulation in the final stages and the most condensed or abbre- 

 viated recapitulation in the initial stages. The development of the individual is 

 everywhere a confirmation of this anticipation, if we allow for the fact that certain 

 stages have greater physiological value than others, and consequently have 

 been favored and kept at the level of need. 6 The cell-stage of the germ is an 

 instance. 



6 This is the work of natural selection, which is omnipresent and efficient in every part of the organic universe. 

 To call for proof of the action of selection is to call for proof of such obvious facts as the struggle for existence and the 

 survival of the fittest. 



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