PLANT AFFINITIES 217 



testing out the environment by supplying new 

 forms that come in competition with the ones 

 already developed. 



LIMITS OF HYBRIDIZING 



But why then, you will perhaps ask, does not 

 the production of new forms between natural 

 species take place so universally as to disturb the 

 entire scheme of organic nature. In point of 

 fact, the zoologist and the botanist are able to 

 describe vast numbers of species, each of which 

 has certain fairly well-defined characteristics and 

 differs in certain definite regards from other 

 forms. 



It is true that the more closely the matter is 

 studied the more commonly varieties are found 

 that manifest characteristics intermediate between 

 those of the supposedly fixed species. But even 

 when these are taken into consideration, it still 

 remains true that the word "species," as applied 

 to a vast number of familiar forms of vegetable 

 and animal life, has a pretty definite and tan- 

 gible meaning. 



How is this possible, if the interbreeding of 

 species is a universal phenomenon? 



The answer is found in the facts (1) that the 

 hybrid forms produced, when species in nature 

 are crossed, for the most part quickly disappear 



