496 Mr. J. D. Dana on Sjtecies. 



at a time in the life of an organic individual, not the whole 

 cycle. 



We may attempt to reach what is called the typical form of a 

 species, in order to make this the subject of a conception. But 

 even within the closest range of what may be taken as typical 

 characters, thei'e are still variables ; and moreover, we repeat it, 

 no one form, typical though we consider it, can be a full expres- 

 sion of the species, as long as variables are as much an essential 

 part of its idea as constants. The advantage of fixing upon 

 some one variety as the typical form of a species is this, — that 

 the mind may have an initial term for the laws embraced under 

 the idea of the species, or an assumed centre of radiation for its 

 variant series, so as more easily to comprehend those laws. 



Again, abrupt transitions and not indefinite shadings have 

 been shown to be the law of nature. In proceeding from special 

 characters to a general species-idea, nature gives us help through 

 her stepping-stones and barriers. In former times, man looked 

 at iron and other metals from the outside only, and searching 

 out their differences of sensible characters, gradually eliminated 

 the general notion of each, by the ordinary logical method of 

 generalization. But science now brings the elements to the line 

 and plummet, and reaches a fixed numbei- for iron and other 

 elements as to chemical combination, &c. By this means, the 

 studying-out of the idea of a species seems almost to have 

 escaped from the domain of logic into that of direct trial by 

 weights and measures. It is no longer the undefined progress 

 of simple reason, with a mere notion at the end, but an appeal 

 to definite measurable values, with stable numbers at bottom, 

 fixed in the very foundations of the universe. So, in the 

 organic kingdoms, where there is, to our limited minds, still 

 greater indetiniteness in most characters, the barrier against 

 hybridity appears to stand as a physical test of species. We 

 are thus enabled, in searching into the nature of a species, to 

 strike from the outside detail to the foundation law. 



The type-idea, as it presents itself to the mind, is no more a 

 subject of defined conception than any mathematical expression. 

 Could we put in mathematical terms the precise law, in all its 

 comprehensiveness, which is at the basis of the species iron, as 

 we can for one of its qualities, that of chemical attraction, this 

 mathematical expression would stand as a representative of the 

 species; and we might use it in calculations, precisely as we can 

 use any mathematical term. So also, if we could write out in 

 numbers the potential nature of an organic species, or of its 

 germ, including the laws of its variables, this expression would 

 be like any other term in the hands of a mathematician ; the 

 mind would receive the formula as an expression for the species. 



