236 THE AMERICAN NATURALIST [Vol. XLII 



of organisms of which the links or component individ- 

 uals are parent and offspring. Objectively, a species is 

 the totality of beings which have come from one stock, 

 in virtue of that most general fact that likeness is trans- 

 mitted from parent to progeny. Among the many defini- 

 tions, that of A. L. Jussieu is one of the briefest and 

 best, since it expresses the fundamental conception of 

 a species, i. e., the perennial succession of similar indi- 

 viduals perpetuated by generation. 



The two elements of species are: (1) community of 

 origin; and, (2) similarity of the component individuals. 

 But the degree of similarity is variable, and the fact of 

 genetic relationship can seldom be established by ob- 

 servation or historical evidence. It is from the likeness 

 that the naturalist ordinarily decides that such and such 

 individuals belong to one species. Still the likeness is 

 a consequence of the genetic relationship; so that the 

 latter is the real foundation of species. 



No two individuals are exactly alike; and offspring of 

 the same stock' may differ (or in their progeny may come 

 to differ) strikingly in some particulars. So two or 

 more forms which would have been regarded as wholly 

 distinct are sometimes proved to be of one species by 

 evidence of their common origin, or more commonly are 

 inferred to be so from the observation of a series of 

 intermediate forms which bridge over the differences. 

 Only observation can inform us how much difference is 

 compatible with a common origin. The general result 

 of observation is that plants and animals breed true 

 from generation to generation within certain somewhat 

 indeterminate limits of variation; that those individuals 

 which resemble each other within such limits interbreed 

 freely, while those with wider differences do not. Hence, 

 on the one hand, the naturalist recognizes varieties or 

 differences within the species, and on the other genera 

 and other superior associations, indicative of remoter 

 relationship of the species themselves. 



