PHYSICAL COISTDITIONS TN CIENESIS OF SPECIES. 401 



ten years ago, hundreds of instances can now he cited of thoroughly 

 proven intergradation, forms then regarded as iniqiiestionahle species 

 being found to be but connected i)hases of one and the same sj)ecific 

 type, which assumes, at remote hx'alities, under the evident action of 

 climatic agencies, phases widely diverse, which gradually merge the 

 one into tlie other through the individuals inhabiting tlie intervening 

 districts. So long as species are based on the absence of intergrada- 

 tion, and biologists have found no other satisfactory criterion for 

 their limitation, there can of course be no passage of one species into 

 another. Let, however, some of the connecting links become extinct, 

 and these now intergi'ading forms Avould l)e resolved into distinct 

 species. In this way insular and other local forms are passing beyond 

 the so-called varietal stage, and species are siniihirly tending to gen- 

 eric distinctness. That varieties may and do arise by the action of 

 climatic influences, and pass on to become species, and that species 

 become, in lilce manner, differentiated into genera, is al)undant]y indi- 

 cated by the facts of geograi)hical distribution and the obvious rela- 

 tion of local forms to the conditions of environment. Tlie ])resent 

 more or less unstable condition of the circuuistances surrounding 

 organi<- beings, together with the known mutations of climate our 

 jDlanet has undergone in past geological ages, points clearly to the 

 agency of physical conditions as one of the chief factors in the evolu- 

 tion of new forms of life. So long as the environing conditions re- 

 main stable, just so long will permanency of character be maintained; 

 but let changes occur, however gradual or minute, and differentiation 

 begins. If too sudden or too great, extinction of many forms nnist 

 result, giving rise to breaks in the chain of genetically connected 

 organisms. In the deep abysses of the sea, where the temperature is 

 low and stable, where the ccmditions of life nuist have remained al- 

 most unvaried since the early geological periods, the same low organ- 

 isuis still exist that were the pre>'ailing forms of life when life first- 

 dawned u[)on the earth. The recent explorations of the depths of the 

 sea have gone far to prove that stability of organic forms is in 

 direct ratio to the stability of the conditions of existence, while the 

 facts of geograjihical distribution show that change of structui'e and 

 diversity of life are directly related to the physical conditions of 

 habitat. 



Note. — Dming the twonty-iiino years that have passed since the original ])nl)- 

 lifation of this article sreat advances liave hcen niaih' in onr knowh'dm' ol' the 

 mannnals and hirds of North America, to which this ])ai)er i)rlniarily relates. 

 Nearly every i)art of the continent has since been explored in great detail by 

 well-trained collectors, eniployiriS new methods and si'eatl.v improved devices, 

 especially for the capture of the smaller rodents and inscctivores, whicii in 



