xv THE PHILOSOPHY OF ZOOLOGY 667 



series of the Ungulata. In many cases such orthogenetic develop- 

 ment has led to excessive growth of parts growth beyond the 

 requirements of the organism an excess which has sometimes, 

 apparently, led to extinction. 



Since natural selection has been judged by many to be inade- 

 quate to account for such straightforward progress in organisms and 

 organs, various other theories of orthogenesis have been put forward. 

 Such of these as merely postulate the existence in organisms of a 

 principle or tendency to develop towards a more perfect condition, 

 fail to reach the standard of admissible scientific theories. Such 

 a supposed tendency is not merely analogous to the tendency of 

 the young organism to grow into the adult form a phenomenon 

 sufficiently difficult to account for by any nexus of causes and 

 effects known to us : it must be something much more, since it 

 must be, not merely a tendency to repeat what has been received 

 in inheritance, but must be in a highly important degree prophetic 

 must, in fact, be a tendency to develop to a point beyond 

 that to which the hereditary impulse reaches. Other theories of 

 orthogenesis rely upon the action of the environment for bringing 

 about the results observed : these have to encounter the same 

 fundamental difficulty as the Lamarckian theory itself the 

 difficulty of explaining how changes in the parents due to the effects 

 of the environment can be impressed on the germ-cells in such a 

 way as to become transmitted to the next generation. But many 

 of those who concern themselves with the study of evolution at 

 the present day, while admitting the absence of any satisfactory 

 theory of the mode of transmission to the germ-cells of changes 

 of organisation in the adult of the nature of acquired characters, 

 are yet inclined to the view that such a transmission does occur, 

 and that without it it is quite impossible to account for the 

 definite adaptive developments that have taken place, especially 

 since there seems to be no convincing explanation, apart from such 

 transmission, of the mode of origin of the first beginnings of 

 structures destined, when further developed, to be of vital im- 

 portance to the organism, but in their early stages not of sufficient 

 value to be capable of determining its survival or extinction. 1 



1 For an ingenious theory of the causes of the initiation and early stages 

 of changes of organisation, see the account of Weismann's theory of germinal 

 *t /erf ion in his work on the Evolution Theory (1904), and his article in 

 Darwin and Modern Science (see Appendix). 



