MEHNERT 351 



which only a few of the elements of the hand or foot come to 

 full development. But whereas in forms with a normally 

 developed hand, e.g. the tortoise and man, all the digits 

 develop and differentiate at about the same rate, in forms 

 which have in the adult reduced digits, e.g. the ostrich and 

 the pig, these vestigial digits undergo a very slow and 

 incomplete differentiation, while the others develop rapidly 

 and completely. He draws a general distinction between 

 organs that are phylogenetically progressive and such as 

 are phylogenetically regressive, and seeks to prove that 

 progressive organs show an ontogenetic acceleration and 

 regressive organs a retardation. 1 The acceleration or re- 

 tardation affects not only the mass-growth of the organs, 

 but also their histological differentiation. 



Now between progression and functioning and between 

 regression and functional atrophy there is obviously a close 

 connection. Loss of function is well known to be one of the 

 chief causes of the degeneration of organs in the individual 

 life, and on the other hand, as Roux has pointed out, all post- 

 embryonic development is ruled and guided by functioning. 

 It is thus in the long run functioning that brings about 

 phylogenetic progression, absence of functional activity that 

 causes phylogenetic regression. This comes about through 

 the transmission of acquired functional characters, a trans- 

 mission which Mehnert conceives to be extraordinarily 

 accurate and complete. 



In general Mehnert adopts the functional standpoint of 

 Cuvier, von Baer, and Roux. His considered judgment as to 

 the phylogenetic value of the biogenetic law closely resembles 

 that formed by von Baer, for he admits recapitulation only 

 as regards the single organs, not as regards the organism as 

 a whole. He has, however, much more sympathy with the 



1 This law was foreshadowed by Reichert in 1837, when he wrote : 

 " We notice in our investigation of embryos of different animal forms 

 that it is those organs, those systems, which in the fully developed 

 individual are peculiarly perfect, that in their earliest rudiments and also 

 throughout the whole course of their development appear with the most 

 striking distinctness" (Miiller's Archiv, p. 135, 1837). See also his 

 Entivick. Kopf. nackt. Amphib., p. 198, 1838. So, too, Ralhke notes how 

 the elongated shape of the snake appears even in very early embryonic 

 stages (Entivick. Natter., p. in, 1839). 



