Mr. J. D. Dana on Species. 495 



In acquiring the first idea of species, we pass, by induction, as 

 in other cases of generalization, from the special details dis- 

 played among individuals to a general notion of a unity of type; 

 and this general notion, when written out in words, we may take 

 as an approximate formula of the species. One system of phi- 

 losophy thence argues that this result of induction is nothing 

 but a notion of the mind, and that species are but an imaginary 

 product of logic ; or at least, that since, as they say (we do not 

 now discuss this point), genera are groupings without definite 

 limits which may be laid ofi" variously by difierent minds, so 

 species are undefined, and individuals are the only realities — the 

 supposed limits to species being regarded as proof of partial 

 study, or a consequence of a partial development of the king- 

 doms of nature. Another system infers, on the contrary, that 

 species are realities, and that the general or type-idea has, in some 

 sense, a real existence. A third admits that species are essentially 

 realities in nature, but claims that the general idea exists only 

 as a result of logical induction. 



The discussion in the preceding pages sustains most nearly 

 the last view, that species are realities in the system of nature 

 while manifest to us only in individuals; that is, they are so 

 far real, that the idea for each is definite, even of mathematical 

 strictness (although not thiis precise in our limited view), it 

 proceeding from the mathematical and finite basis of nature. 

 They are the units fixed in the plan of creation ; and individuals 

 are the material expressions of those ideal units. 



At the same time, we learn, that while species are realities 

 in a most important and fundamental sense, no comprehensive 

 type-idea of a species can be represented in any material or 

 immaterial existence. For while a species has its constants, it 

 has also its variables, each variable becoming a constant so far 

 only as its law and limits of variation are fixed; and in the 

 organic kingdoms, moreover, each individual has its historic 

 phases, from the germ through the cycle of growth. The 

 general idea sought out by induction, therefore, is not made up 

 of mvariables. Limited to these, it represents no object, class 

 of objects, or law, in nature. The variables are a necessary 

 complement to the invariables ; and the complete species-idea 

 is present to the mind, only when the image in view is seen to 

 be ever changing along the lines of variables and development. 

 Whatever individualized conception is entertained, it is evidently 

 a conception of the species in one of its phases, — that is, under 

 some one specific condition as to size, form, colour, constitution, 

 &c., as regards each part in the structure, from among the many 

 variations in all these respects that are possible : mind can pic- 

 ture to itself individuals only and not species, and one phase 



