176 Darwin and Embryology 



is negative. The recapitulation theory originated as a deduction 

 from the evolution theory and as a deduction it still remains. 



Let us before leaving the subject apply another test. If the 

 evolution theory and the recapitulation theory are both true, how 

 is it that living birds are not only without teeth but have no rudiments 

 of teeth at any stage of their existence ? How is it that the missing 

 digits in birds and mammals, the missing or reduced limb of snakes 

 and whales, the reduced mandibulo-hyoid cleft of elasmobranch fishes 

 are not present or relatively more highly developed in the embryo 

 than in the adult ? How is it that when a marked variation, such 

 as an extra digit, or a reduced limb, or an extra segment, makes its 

 appearance, it is not confined to the adult but can be seen all through 

 the development ? All the clear evidence we can get tends to show 

 that marked variations, whether of reduction or increase, of organs 

 are manifest during the whole of the development of the organ and 

 do not merely affect the adult. And on reflection we see that it could 

 hardly be otherwise. All such evidence is distinctly at variance with 

 the theory of recapitulation, at least as applied to embryos. In the 

 case of larvae of course the case will be different, for in them the 

 organs are functional, and reduction in the adult will not be accom- 

 panied by reduction in the larva unless a change in the conditions 

 of life of the larva enables it to occur. 



If after 50 years of research and close examination of the facts 

 of embryology the recapitulation theory is still without satisfactory 

 proof, it seems desirable to take a wider sweep and to inquire whether 

 the facts of embryology cannot be included in a larger category. 



As has been pointed out by Huxley, development and life are 

 co-extensive, and it is impossible to point to any period in the life of 

 an organism when the developmental changes cease. It is true that 

 these changes take place more rapidly at the commencement of life, 

 but they are never wholly absent, and those which occur in the later 

 or so-called adult stages of life do not differ in their essence, however 

 much they may differ in their degree, from those which occur during 

 the embryonic and larval periods. This consideration at once brings 

 the changes of the embryonic period into the same category as those 

 of the adult and suggests that an explanation which will account for 

 the one will account for the other. What then is the problem we are 

 dealing with ? Surely it is this : Why does an organism as soon as it 

 is established at the fertilisation of the ovum enter upon a cycle of 

 transformations which never cease until death puts an end to them ? 

 In other words what is the meaning of that cycle of changes which all 

 organisms present in a greater or less degree and which constitute the 

 very essence of life ? It is impossible to give an answer to this question 

 so long as we remain within the precincts of Biology — and it is not 



