Eeactions and Products in Inteespecific Crosses 181 



duces blue rhomboidal crystals when CUSO4 and SHgO are combined. Not that 

 it is the same, but that it has the same physical principles of operation and causes 

 of the resultant characters, which are after all only temporary states of stability 

 in the ceaseless integrations and dissipations of matter that characterize the 

 progression of organisms in nature. 



In this crossing of species from nature I have presented for the most part only 

 data that tested to the limit of present operations the truth and application of 

 the Mendelian reaction. I have found no blends, no instances, totally at vari- 

 ance with the principles of action of this test of gametic composition, and while 

 it is true that there remain in this account lines that have arisen whose entire 

 composition has not been entirely solved, or whose production is not entirely 

 clear, still no instance has been found that has not at least given exact evidence 

 that the Mendelian reaction and the principle of gametic factorial agents are not 

 present and operating, even though at present obscured by unsolved portions of 

 the problem. This latter is, however, open to continued test of experiment, and 

 empirically it is possible to progress with certainty in the solution of these com- 

 plications. 



The literature is abundantly supplied with instances of the rise of pure-breed- 

 ing and stable races as the product of an initial-species cross, and in them the 

 fact of their pure breeding in following generations has been the criterion of 

 purity, blending, and kindred assertions concerning them. Many Mendelian 

 workers, Bateson especially, has expressed doubt of this being true, and while 

 it may be true in some instances, I am at present not informed of any instances 

 where the testing of them has been even a tenth part as rigorous as that given 

 in similar lines in my experiment; and so, on the basis of experience, in this 

 series we may well ask that there be other tests applied than those of " breeding 

 true " and of the simpler routine Mendelian reactions. 



In my experience this Mendelian reaction has thus far stood the test of the 

 most hostile treatment I could give it, and everywhere it has proved to be certain 

 and precise, correct, and when one dissociates it from misconceptions and the 

 drag of biological units it takes on a new significance and in investigation 

 becomes one of the most important tools of progress that we possess to-day. The 

 probable nature of this reaction is discussed in Chapter IV. 



THE PROBLEMS OF THE CROSSING OF SPECIES AND SPECIES PRODUCTION. 



In spite of assertion that the problems of evolution are those of character 

 origin, it must still be admitted that the problem of the origin of the groups in 

 nature that are called species is a real problem for investigation. The older 

 writers upon this problem have interpreted and argued the situation over and 

 over again, with the well-known results that the problem of the origin of these 

 groups is by no means solved, nor is the probable mechanism of production much 

 more than a matter of opinion. 



Considerations of origins have largely centered about single individuals as 

 the progenitor of the group, or at least a pair, in the thoughts of those who con- 

 ceived of species rise through saltations, while the separation or differentiation 

 of a group by slow changes remains largely a pure speculation. It is not at 

 present possible to decide how any given species in nature arose, nor how long 

 it has existed, aside from the evidences derived from fossil remains, which are 

 on the whole fragmentary and insecure. All that is possible is to discover 



