The Theory of Evolution 85 



to a new environment. Others change with the environ- 

 ment. It is not improbable that the varieties are of a dif- 

 ferent kind in these two cases, as shown by their different 

 behavior when put under new and different surroundings. 

 The variety that owes its peculiarities, not to the immediate 

 environment, but to some internal condition independent of 

 the surroundings, is recognized by some biologists as a 

 smaller species. Such species appear to be commoner in 

 plants than in animals, although it is possible that this only 

 means that more cases have been found by the botanists, 

 owing to the greater ease with which plants can be handled. 

 These smaller species, in contradistinction to the ordinary 

 Linnaean species, differ from the latter in the smaller amount 

 of differences between the groups, and probably also in that 

 they freely interbreed, and leave fertile descendants ; but 

 whether this is only on account of the smaller differences 

 between them than between larger species, or because of 

 some more fundamental difference in the kind of variation 

 that gives rise to these two kinds of groups, we do not know. 



These smaller species, or constant varieties, as we may call 

 them, may be looked upon as incipient Linnaean species, 

 which, by further variations of the same, or of other sorts, 

 may end by giving rise to true species. A genus composed 

 of several species might be formed in this way, and then, if 

 each species again broke up into a number of new groups, 

 each such group would now be recognized as a genus, and 

 the group of genera would form a family, etc. The process 

 continuing, a whole class, or order, or even phylum, might be 

 the result of this process that began in a single species. 



But we must look still farther, and inquire whether the 

 start was made from a single individual, that began to vary, 

 or from a number of individuals, or even from all the indi- 

 viduals, of a species. If we suppose the result to depend 

 on some external cause that affects all the individuals of a 

 species alike, then it might appear that the species, or at 



