646 ZOOLOGY 



SECT. XV 



dages undergo progressive modification, so that each pair becomes 

 specially adapted for the performance of particular functions. 

 Moreover, parallel and evidently independent lines of orthogenetic 

 development are in many cases traceable in separate groups. As 

 examples may be mentioned the series of stages in the development 

 of complex eyes from simple rudiments, which are observable in 

 the Annulata, the Arthropoda, and the Chordata : the parallel and 

 quite unconnected stages in the reduction of the digits, leading 

 to the greater perfection of the limbs as running organs, to be 

 traced in the Perissodactyle and Artiodactyle series of the Ungulata. 

 In many cases such orthogenetic development 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 inadequate 

 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 ortho- 

 genesis rely upon the action of the environment for bringing about 

 the results observed : these have to encounter the same funda- 

 mental 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 experimental 

 evidence in favour of the heritable character of various changes 

 caused by external factors has been recently accumulating. And 

 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 trans- 

 mission, of the mode of origin of the first beginnings of structures 

 destined, when further developed, to be of vital importance to the 

 organism, but in their early stages not of sufficient value to be 

 capable of determining its survival or extinction. 



