CH. x] A. NUMERICAL 91 



monly, some difference in the conditions, better than did their 

 immediate ancestor, which must have been suited to the condi- 

 tions to survive and reproduce. This would most probably mean 

 some difference in the physical conditions, especially of climate 

 or of soil, or in physical differences due to the presence of other 

 organisms, such as greater shade, greater demand for some 

 chemical constituent of the soil, or other thing. But ivhy should 

 a change in soil, or in climate, or in biological surroundings, un- 

 less perhaps it were very strongly marked, involve any morpho- 

 logical change? It is very difficult to see any connection between 

 these things. 



Unless by some accidental happening, or in the rare case of a 

 "pure stand" — a solitary species occupying a large area — the 

 surroundings made by other plants would be continually variable. 

 Weather also is changeable, and unless a species were suited 

 from its birth to this fact, it would have a very poor chance of 

 survival in any case. Soil varies from one spot to another, and 

 so on. Unless variation in the conditions went continuously 

 in the same direction, as for example in a change of climate (not 

 of weather), it is very difficult to see why variations in the 

 morphological characters of the plant should go always in the 

 same direction, as is required if they are to be added up to 

 make specific differences. And it is difficult to see why, for 

 example, there should be any need for change at all in a species 

 that occurs, as do most species, principally in one association of 

 plants. 



But to get increasing numbers of species, one species must (at 

 any rate very often) give rise to two or more, not simply to one 

 new one, unless, as on the theory of differentiation, the parent 

 survive as well as the offspring. But upon the theory of gradual 

 adaptation, to get two or more species from one without losing 

 them by intercrossing in the early stages, one must have dif- 

 ferent conditions in different parts of the range of the same 

 parent species. In other words, it must occupy a fairly large 

 area to get into such variety, and this is the basis of the explana- 

 tion of the local species as relics, though they far outnumber the 

 widely distributed ones, even in the most "successful" genera. 



But if all the local species are failures, where does the increase 

 in number come from? Even in his own diagram (6, p. 90) 

 Darwin begins with eleven species, which at the next stage 

 become reduced to seven, the rest disappearing. At an indefi- 

 nitely later stage, shown in faint lines, they have increased to 



