182 Darwin and Embryology 



least as long as it is at the present time. The qualification implied 

 by the words in italics is necessary, for it is clearly possible that the 

 external conditions then existing were not suitable for the production 

 of all the stages of the potential life-history, and that what we call 

 organic evolution has consisted in a gradual evolution of new en- 

 vironments to which the organism's innate capacity of change has 

 enabled it to adapt itself. We have warrant for this possibility in 

 the case of the Axolotl and in other similar cases of neoteny. And 

 these cases further bring home to us the fact, to which I have already 

 referred, that the full development of the functional reproductive 

 organs is nearly always associated with the final stages of the life- 

 history. 



On this view of the succession of characters in the life-history of 

 organisms, how shall we explain the undoubted fact that the develop- 

 ment of buds hardly ever presents any phenomena corresponding to 

 the embryonic and larval changes ? The reason is clearly this, that 

 budding usually occurs after the embryonic stage is past; when the 

 characters of embryonic life have been worked out by the machine. 

 When it takes place at an early stage in embryonic life, as it does in 

 cases of so-called embryonic fission, the product shows, either partly 

 or entirely, phenomena similar to those of embryonic development. 

 The only case known to me in which budding by the adult is 

 accompanied by morphological features similar to those displayed 

 by embryos is furnished by the budding of the medusiform spore-sacs 

 of hydrozoon polyps. But this case is exceptional, for here we have 

 to do with an attempt, which fails, to form a free-swimming organism, 

 the medusa; and the vestiges which appear in the buds are the 

 umbrella-cavity, marginal tentacles, circular canal, etc., of the medusa 

 arrested in development. 



But the question still remains, are there no cases in which, as 

 implied by the recapitulation theory, variations in any organ are 

 confined to the period in which the organ is functional and do not 

 affect it in the embryonic stages ? The teeth of the whalebone whales 

 may be cited as a case in which this is said to occur; but here the 

 teeth are only imperfectly developed in the embryo and are soon 

 absorbed. They have been affected by the change which has 

 produced their disappearance in the adult, but not to complete 

 extinction. Nor are they now likely to be extinguished, for having 

 become exclusively embryonic they are largely protected from the 

 action of natural selection. This consideration brings up a most 

 important aspect of the question, so far as disappearing organs are 

 concerned. Every organ is laid down at a certain period in the 

 embryo and undergoes a certain course of growth until it obtains 

 full functional development. When for any cause reduction begins. 



