Mr. J. D. Dana on Species. 493 



may be exposed. The new germ, moreover, takes peculiarities 

 from the parent, or from the circumstances to which its ancestry 

 had been exposed during one or more preceding generations. 



There is then a fixed normal condition or value, and around 

 it librations take place. There is a central or intrinsic law which 

 prevents a species from being drawn off to its destruction by any 

 external agency, while subject to greater or less variations under 

 extrinsic forces. 



Liability to variation is hence part of the law of a species ; 

 and we cannot be said to comprehend in any case the complete 

 idea of the type until the relations to external forces are also 

 known. The law of variables is as much an expression of the 

 fundamental equalities of the species in organic as in inorganic 

 nature ; and it should be the great aim of science to investigate 

 it for every species. It is a source of knowledge which will yet 

 give us a deep insight into the fundamental laws of life. Varia- 

 tions are not to be arranged under the head of accidents; for 

 there is nothing accidental in nature ; what we so call, are ex- 

 pressions really of profound law, and often betray truth and law 

 which we should otherwise never suspect. 



This process of variation is the external revealing the internal, 

 through their sympathetic relations ; it is the law of universal 

 nature reacting on the law of a special nature, and compelling 

 the latter to exhibit its qualities ; it is a centre of force mani- 

 festing its potentiality, not in its own inner working, but in 

 its outgoings among the equilibrating forces around, and thus 

 offering us, through the known and physical, some measure of 

 the vital within the germ. It is therefore one of the richest 

 sources of truth open to our search. 



The limits of variation, it may be difficult to define among 

 species that have close relations. But being sure that there are 

 limits, — that science, in looking for law and order written out 

 in legible characters, is not in fruitless search, we need not 

 despair of discovering them. The zoologist, gathering shells or 

 mollusks from the coast of eastern America and that of Japan, 

 after careful study, makes out his lists of identical species, with 

 the full assurance that species are definite and stable existences ; 

 and he is even surprised with the identity of characters between 

 the individuals of a species gathered from so remote localities. 

 And as he sees zoological geography rising into one of the 

 grandest of the sciences, his faith in species becomes identified 

 with his faith in nature and all physical truth. 



If then we may trust this argument from general truths to 

 special, — general truths I say, for general principles as far as 

 estabhshed are truths — we should conceive of a species from 

 the potential point of view, and regard it as — 



