10 THE FOUNDATIONS OF ZOOLOGY 



in their physical development, is dependent, to a great degree at 

 least, on some constitutional stimulus which is afforded by the 

 changes which take place in the reproductive organs. 



The existence of rudimentary organs and provisional larval 

 stages is one of the most suggestive facts in the whole range of 

 zoology, and the evidence that these things are a record of past 

 history seems conclusive ; although those who hold that their 

 existence is accounted for by the discovery that they are a 

 "recapitulation" add nothing, after all the centuries, to Aristotle's 

 declaration that they are "for a token." 



They who are most convinced that the historical significance 

 of these structures is an adequate explanation of their presence, 

 are also most emphatic in their repudiation of teleology, and in the 

 rejection of the belief of Louis Agassiz, that they are part of the 

 language in which the Creator tells us the history of creation ; 

 yet the assertion that their history accounts for their existence is 

 as teleological as anything in Paley. 



They who believe that inheritance is not the transmission of re- 

 sponsive actions, but the transmission of an adaptive mechanism, and 

 that each change which enters into the history of development is a 

 response to a stimulus, will have no difficulty in understanding that 

 organs which were once adjusted to the external world may, after 

 this adjustment has lost its meaning, be still retained, because they 

 furnish physiological stimuli, which excite developmental changes 

 in the organic mechanism. 



If a physiological stimulus from the male reproductive organs 

 excites the growth of weapons of defence, would the preservation 

 of rudiments of these organs, by natural selection, for this useful 

 purpose, be anything more than might be expected; even if some 

 change in the method of reproduction should make their primary 

 function useless.' 



Is there any evidence that any change which is due to nature, 

 from the segmentation of the ^gg to old age, ever takes place 

 without a stimulus, or are the actions which are due to nature 

 beneficial, except so far as the environment is, on the average, 

 like the ancestral environment } Since the gentle stimulation of 

 the lips and tongue has been associated, in the past history of 

 human infants, with the presence of milk which may be extracted 



