234 EVOLUTION 



to make way for the other more apparent and 

 better known factors. The first indispensable 

 factor, and perhaps the most important in any 

 case, in every transformation is the physical 

 nature of the organism itself." 



This inquiry into the organismal springs 

 of variation must lead us far. For Weismann 

 these have led especially into his subtle 

 studies of the germ-plasm; but obviously 

 also they involve a fresh survey of the leading 

 types of variation as we see them developed 

 by plant and animal forms. Naturalists are 

 no longer so much setting out from the analogy 

 of human selection upon domestic animals and 

 cultivated plants, and reasoning from the 

 accumulation of their varietal differences up 

 to what seem to correspond to species or some- 

 times even genera in Nature, and thence 

 arguing on LyelPs uniformitarian principle, 

 for the analogous cumulative natural selection 

 through geological time, of the characters of 

 larger groups, genera, orders, classes and the 

 rest. We have simply jiow to group our types 

 of variation, and to consider them from the 

 standpoint of general physiology as far as we 

 know it, and independently of these fascinat- 

 ing hypotheses of agriculture and geology. 



VEGETATION AND REPRODUCTION, AND 

 THEIR ANTITHESIS. The largest view of physi- 



