80 



THE AMERICAN NATURALIST 



[Vol. XLII 



tinctness to some form of isolation or segregation, and 

 that, broadly speaking, with occasional exceptions, given 

 any form of animal or plant in any region, the nearest 

 related form is not to be found in the same region nor 

 in a remote region, but in a neighboring region, separated 

 from the first by a barrier of some sort, not freely 

 traversable. 



A law, that is, an observed relation of cause and effect 

 is not invalidated by the presence of other effects due 

 to other causes, in the same environment. The actual 

 conditions in nature are everywhere not products of single 

 and simple forces, but resultants of many causative in- 

 fluences, often operative through the long course of the 

 ages. 



It may be urged that these geminate groups or forms 

 are not true species, because they often intergrade one 

 into another, and they would probably be lost by inter- 

 mingling if the barriers were removed. It is sometimes 

 claimed that only physiological tests of species can be 

 trusted, as true species will not blend and their hybrids, 

 if formed, will be sterile. All this is purely hypothetical 

 and impracticable to the systematic zoologist, and not 

 of much value to the botanist. Closely related species 

 can usually be readily crossed. As the relation becomes 

 less close, partial sterility of all grades and then total 

 sterility appear. 



Species as we find them in nature are real species if 

 that term has any definition. And real species have, as 

 a rule, indefinite boundaries, shading off into subspe- 

 cies, geminate species, ontogenetic forms and the like. 

 And if we are to understand the significance of nature, 

 we have to describe these facts and relations as they 

 actually are. Then we have to find out what changes we 

 can work in individuals and in species by such alterations 

 of conditions as experiment can give. 



We do not know actually any species of animal or plant 

 until we know all changes that would take place in its 

 individuals under all conditions of environment. 



