1864.] 407 



mix sexually together, or if geographically separated, that they would 

 not do so, supposing them to be placed in juxtaposition." According 

 to this view of what Darwin calls " the undiscovered and undiscovera- 

 ble essence of the term species," {Orig. Spec. p. 421,) it is immaterial 

 whether the distinctive characters be slight or strong, so that they be 

 perfectly constant. But as many naturalists are of opinion, that to con- 

 stitute a distinct species it is nece.ssary that the distinctive characters 

 should be tolerably strong, it will be better to distinguish Phytophagic 

 Species by this particular denomination, and not confound them with 

 the general mass of what are known as distinct species. After all, the 

 difference of views on this subject is a difference only in words and not 

 in things. I consider as species all forms which do not habitually in- 

 termix in a state of nature — as according to the definition of the term 

 Phytophagic Species do not — the absence of intermediate grades being, 

 as a general rule, taken as the criterion of the species not habitually 

 intermixing in a state of nature. Others require in addition, that the 

 distinctive characters should be of a certain type, which is left to be fixed 

 and defined in each particular genus by certain varying and somewhat 

 indefinite rules. It is evident, therefore, that the term "species" is 

 used here in two different senses, and to avoid ambiguity it is necessary 

 to distinguish the doubtful and disputed forms by some particular 

 name. 



It may be asked why the process by which PhytopRagic Species are 

 formed is not reiterated on all hands, till Nature becomes a Babel of con- 

 fusion and the number of distinct species equals the grains of sand on the 

 sea-shore. The answer is simple. There are two great antagonistic 

 forces in Nature, the Law of Variation, causing individuals of almost 

 all species to assume occasionally abnormal characters or abnormal pro- 

 pensities, and what may be called the L.\w of Assimilation, which, 

 by the intercrossing of these abnormal individuals and their descendants 

 with the normal type, gradually in successive generations softens down, 

 eliminates and extirpates whatever is strange and peculiar in tlicni. 

 Thus, American families of the pure Caucasian race, which intercrossed 

 in a single instance many generations since with the Bed Indian, have 

 already, by successive intercrosses with the White Bace, completely 

 eliminated all traces of Indian blood. It can only be in very rare cases 

 indeed, that the process which I have been describing can be carried 



