LAW OF GEMINATE SPECIES JORDAN 121 



hills by Rubus alle ghaniensis and in the far west by Rubus mtijolius, while 

 still other species surround the world, taking its place in Europe and in Asia. 



We may, therefore, say that with plants as well as animals geminate 

 species as above denned owe their distinctness 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 travers- 

 able. 



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 influences, often operative 

 through the long course of ages. As a rule, also, related species in almost 

 every group are connected by a fringe of intergradations we call subspecies. 

 If barriers are sharply defined geminate species are sharply defined, also. 

 If barriers are diffuse, we find geographical subspecies connecting them, 

 either wholly or in part. There is no difference between a subspecies and a 

 true species except that which is involved in sharpness of definition. If the 

 barrier cannot be crossed, the species dependent on the barrier is well defined 

 and therefore unquestionable, however small the elements of difference. A 

 subspecies, if real, is always based on some matter in geographical distribu- 

 tion. 



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 intermingling 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 hypo- 

 thetical and impracticable to the systematic zoologist, and not of much value 

 to the botanist. Interbreeding is no test of species. Closely related species 

 in almost any group of plants or animals can usually be readily crossed. As 

 the relation becomes less close, partial sterility of all grades and then total 

 sterility appear. 



If the term species has any meaning at all, those species we find in nature 

 are real species. Nothing can be more real than that which actually exists. 

 And real species have, as a rule, indefinite boundaries, shading off into sub- 

 species, geminate species, ontogenetic forms and the like. In these eccen- 

 tricities we must humor them. As Darwin observed, these peculiarities are 

 fascinating to us "as speculatists" however "odious" they are to us "as sys- 

 tematists." And if we are to understand the significance of nature, we have 



