302 THE LIFE OF THE PLANT 



of a single species, and when with two independent 

 species. Not at all : they not only give nothing of 

 the kind, but neither can they do so, because species as 

 well as varieties defy definition. We shall soon see 

 why, starting with the a priori conviction that one species 

 cannot arise from another, those who maintain their 

 immutability generally follow a rule which says that 

 two forms, connected by transitional forms, however 

 unlike they may themselves be, cannot be acknow- 

 ledged as independent species. Guided by the same 

 rule, however, the opponents of the theory point to 

 species between which transitional forms have been 

 discovered. The others answer : ' This means that we 

 have been mistaken, that we have taken for a species 

 something which in the main is a variation.' Obviously 

 arguing in circles like this keeps up appearances. Affirm- 

 ing that species are invariable, they may easily assign the 

 term variety to every variable form, in the absence of any 

 positive sign which would differentiate a species from a 

 variety. There was a time, however, when those who 

 maintained the immutability of species did cherish the 

 hope of establishing the difference between species and 

 varieties upon some positive physiological feature. A 

 conviction arose that all the representatives of a species, 

 all its variations, however different they may be amongst 

 themselves, can intercross, producing hybrids capable of 

 further reproduction ; and that, on the contrary, species 

 are unable to produce hybrids, or, if they happen to do 

 so, the hybrids are bound to be sterile. It has even 

 been presumed that Nature has of set purpose made 

 species invariable. After having produced a certain 

 number of specific forms Nature has taken the trouble 

 to preserve them for ever invariable, by preventing 

 them from the possibility of varying as the result of 

 their forming hybrids. When we discussed fertilisation, 

 we stated facts which definitely contradicted this theory. 

 We have seen that fertilisation by means of the pollen 



