400 PHYSICAL COKDITTONS IN GENESIS OF SPECIES. 



discussions have arisen, and one respecting which the most discordant 

 opinions have been hehi by naturalists equally eminent in their re- 

 spective fields of research. The amount and kind of difference neces- 

 sary to characterize a species has been variously defined ; forms that 

 some have considered as specific others have regarded as merely varie- 

 ties, and the reverse. In certain groups of organisms intermediate 

 forms have been constantly met with, constituting steps of easy inter- 

 gradation between quite diverse types. Such forms have been and still 

 are held by some writers as varieties of a single species and by others 

 as constituting a group (genus or subgenus) of distinct but nearly 

 related species. Through the frequent discovery of such intergrada- 

 tions, how^ever, the instability of so-called '" species " lias been made 

 manifest and the contrary doctrine of the stability or fixity of species 

 refuted. Indeed, naturalists now generally agree that the terms 

 " variety,"' " species," " genus."" " subgenus,"' "" family,"" '' subfamily,'" 

 " superfamily,"" and the like, are but conventional and more or 

 less arbitrary designations for different degrees of differentiation — 

 convenient fornuda^ for the expression of general facts in biology. 

 Not a few high authorities even maintain that the differences which 

 characterize these several groups are of the same nature, differing 

 only in degree, in opposition to others who hold that they are based 

 on different categories of structure, or on differences of I'lnd rather 

 than of degree. The falsity of the latter view is shown more and 

 more clearly Avith the increase of our knowledge of the structure and 

 affinities of animals. 



While formerly species were considered as necessarily characterized 

 either by differences of a particular kind, or by a certain amount of 

 difference, the present tendency is to regard neither as a sufficient 

 critericm, the test of specific diversity being merely absence of inter- 

 gradation, in other words, breaks in the continuity of closely allied 

 beings. Local races, or geographical forms, are thrown together 

 under one specific designation whenever they are found to intergrade, 

 however diverse may be their extreme phases of differentiation. The 

 term species is now made to cover groups which w^ere, not many years 

 since, frequently regarded as subgenera, or even genera, the forms 

 then supposed, in numberless instances, to be " good species,"" now 

 ranking merely as subspecies. The reduction in the number of 

 species has necessarily entailed a considerable reduction in the number 

 of currently accepted genera, which in turn are limited by liiati rather 

 than liy any given amount or particular kind of difference. It was 

 formerly urged against the theory, of evolution that its advocates 

 could ])oint to no instance of the gradual change of one species into 

 another, and that, until this was done, the theory Avas untenable. 

 Among the species of North American vertebrates recognized as valid 



