180 THE AMERICAN NATUBALIST [Vol.XLIX 



fact, which appear to have no direct relation whatever 

 to the well-being of the organism in which they appear, 

 bnt to be purely accidental. Such mutations are, of 

 course, inherited ? and, inasmuch as the great majority of 

 specific characters appear to have no adaptive signifi- 

 cance, it seems likely that mutation has had a great deal 

 to do with the origin of species, though it may have had 

 very little to do with progressive evolution. 



Similarly with regard to hybridization, we know that 

 vast numbers Of distinct forms, that breed true, may be 

 produced in this way, but they are simply due to recom- 

 binations of mutational characters in the process of am- 

 phimixis, and have very little bearing upon the problem 

 of evolution. If we like to call the new groups of indi- 

 viduals that originate thus "species," well and good, but 

 it only means that we give that name, as a matter of 

 convenience, to any group of closely related individuals 

 which are distinguished by recognizable characters from 

 the individuals of all other groups, and which hand on 

 those characters to their descendants so long as the con- 

 ditions remain the same. This, perhaps, is what we 

 should do, and just as we have learned to regard indi- 

 viduals as the temporary offspring of a continuous 

 stream of germ-plasm, so we must regard species as the 

 somewhat more permanent but nevertheless temporary 

 offshoots of a continuous line of progressive evolution. 

 Individuals are to species what the germ-plasm is to in- 

 dividuals. One species does not arise from another 

 species, but from certain individuals in that species, 

 and when all the individuals become so specialized as to 

 lose their power of adaptation, then changes in the en- 

 vironment may result in the extinction of that line of 

 descent. 



It is scarcely necessary to point out that no explana- 

 tion that we are able to give regarding the causes of 

 either phylogenetic or ontogenetic evolution can be com- 

 plete and exhaustive. Science can never hope to get to 



