32 PHYSICAL INVESTIGATION. 



by domestic animals and man. The range of variation of evei 

 species seems to be peculiar to each, and governed by specii 

 laws. Hence, it proves nothing when Agassiz says, the chim- 

 panzee and the gorilla differ no more from each other than 

 Mandingo and the Gruinea Negro, and both do not differ more 

 from the orang than the Malay or European from the Negro : 

 if the former are to be considered as belonging to different 

 species, the latter should be considered equally so. That par- 

 ticular laws of formation govern individual species of animals 

 is indicated by their different capacity for the production of 

 hybrids. This would be certain if Desmoulins were right in 

 his assertion that the diversities of various species, and their 

 sphere of variation, diminishes the more they approach man. 

 The fallaciousness of drawing conclusions from the analogy of 

 one species to another has recently been pointed out by 

 Lucas, 1 who proves, by many examples, how extremely differ- 

 ent, quantitatively and qualitatively, is the power of resistance 

 in different races to external influences in regard to the trans- 

 formation of races. Some other instructive examples are 

 furnished by Giebel. 2 They show that similar deviations in 

 different genera of animals are of very variable importance, e ' as 

 in one family or genus one or another organ has obtained a 

 particular significance for the whole organism," so that fre- 

 quently characters or groups of characters which are essential 

 to one constitute in the other no fixed specific characters, but 

 vary greatly. 



In condensing the results of our investigation regarding the 

 definition of species, we have found that it designates those 

 types permanent which are transmitted by propagation. We 

 were induced to separate the questions of unity of species and 

 unity of descent on the ground that the same assemblage of 

 constant characters may belong originally to distinct stocks ; 

 and we could not, therefore, consider unity of descent as neces- 

 sary to our definition of species. If, thus, separate descent 

 was no valid proof for difference of species, unlimited prolifi- 



1 "Traite philosophique et physiol. de 1'heredite naturelle," ii, p. 116, 1857 ; 

 and Nusard, ibid., ii, p. 452. 



2 Loc. cit., p. 



