THE ORIGIN OF NEW FORMS 187 



or they may be modified under proper conditions, returning to the 

 form from which the parent sprang. Forms which remain true 

 to the parent type under different conditions are said to be 

 constant, while those that revert to the type preceding the parent 

 are termed inconstant. 



For a great many years species were supposed to be constant, 

 but varieties and forms inconstant. Constancy was rarely made 

 a matter of experiment, however, and was practically never used 

 in connection with the naming of new species. In consequence, 

 many species of the manuals are not constant, and many varieties 

 and forms are. The distinction between these three disappears 

 accordingly. It has slight meaning to distinguish one new form 

 as a species and another as a variety. The use of either term in 

 any exact scientific manner is difficult until the present so-called 

 species are thoroughly examined experimentally, and new criteria 

 are established as a result. The common usage of descriptive 

 botany is to term new forms species, regardless of the way in 

 which they originate or the amount of difference they show. It 

 is necessary to distinguish different kinds of species, or to make 

 a definite distinction between species and other forms. The 

 basis for either procedure must be experiment and not observation 

 merely, though the latter is often an aid. 



Constancy is in nowise a test of evolution, though it plays an 

 important part in the arrangement of the new forms that arise. 

 The way in which new forms originate determines whether they 

 shall be constant or not, i.e., constancy is itself a result of evolu- 

 tion, rather than a factor in it. IMoreover, it seems very probable 

 that constancy is directly influenced Ijy the habitat. A shade 

 form that has sprung from a sun plant usually reverts at once 

 to the original form if the seeds of the first generation are grown 

 in the sun. This reversion seems to take place more slowly after 

 a number of generations, and it is probable that it would be 

 slower or more incomplete after a hundred or a thousand genera- 

 tions. 



203. Origin by descent before Darwin, Before the appearance 

 of Darwin's "Origin of Species" in 1859, it was commonly be- 

 lieved that genera and species were the result of si)ecial creati\-e 

 acts. Varieties, on the contrary, were supjwsed to arise from 

 species through the influence of external conditions. This was 

 the view held by Linnaeus, whose authority was such as to cause 



