RELATIONSHIP OF SPECIES. 97 



which occur in species, and which are so apparent to the eye ; 

 no matter that the germ-plasm initiates these changes, or is 

 affected by them (such as they are), they are more apparent than 

 real, that is, as indicating any definite change in the parent 

 germ-plasm, in so far as to any change in powers of inter- 

 breeding, conception, and the begetting of young. For instance, 

 these " fluctuating " and " mutating " animals are almost 

 invariably quite fertile with recent similar developments of each 

 other, as shown by the interbreeding of some wild and all (as far 

 as I know) domestic varieties. Therefore it seems feasible to 

 suppose that the germ-plasm changes only gradually, as sug- 

 gested by the results given in the table of hybridisation, showing 

 the gradual reduction of fertility given above, and that there are 

 not generally any sudden changes or "jumps" in this germ- 

 plasm in the nature of anything akin to mutation, such as are 

 visible as structural changes to the eye. If, as suggested by 

 Weismann, the physiological unit of the germ-plasm is built up 

 architecturally (any alteration in design may possibly be a later 

 development denoting greater changes) or whether, as more 

 recent investigators suggest, the changes are due chemically to 

 ferments, it is still possible the change is a gradual one and 

 rarely effected suddenly, even if, as De Vries holds, there are 

 times of violent activity in the life of a species, during which it 

 tends to throw off mutations. As far as I can reason, the 

 examples I have quoted appear to be evidence sufficiently strong 

 to show gradual changing of the germ-plasm (more or less rapid 

 though it may be through such stimulating effects as changing 

 environment and food), no matter how spontaneous or important 

 the visible jump may be. I believe that the inward invisible 

 germ-plasm relationship (which one might expect would be 

 reciprocated in blood tests) is most generally the true evidence of 

 relationship as opposed to that entirely founded on every struc- 

 tural difference, great or small. This does not mean, however, 

 that structural evolution and germ-plasm changes do not go 

 hand in hand, for it would seem that it is only generally in the 

 closer relationship of the various groups that one, so to speak, 

 gets ahead of the other. 



For instance, let us assume two forms evolute out, both, 

 perhaps, still largely feeding on the same substances, the evolu* 

 Zool. 4th Her. vol XVIII., March, 1914. i 



