THE IMPORTANCE OF PERSISTENCE 63 



Professor Lankester in former years l published the 

 suggestion that the term species should be limited to 

 a group which includes all the forms derived from 

 common ancestors within human experience, or inferred 

 to be so derived within the possible period of human 

 observation. Thus if the common ancestry of two 

 forms has to be traced back to a period beyond the 

 late prehistoric times (or beyond any other arbitrary 

 line which is agreed upon), then they are not members 

 of the same species. Professor Lankester is the first 

 to admit that the practical application of this as of 

 every other conception of species would very often 

 mean a great deal more than we can prove, in fact, 

 hypothesis. 



It is evident that Darwin regarded constancy of 

 form as an important criterion of a species. We re- 

 cognize this in the definition I have quoted from the 

 Origin (see p. 47), and it is stated with even greater 

 force in the following passage, where persistence is placed 

 beside other distinguishing marks of a species and given 

 the pre-eminence. In a letter to Hooker (October 22, 

 1864) Darwin says : ' I will fight you to the death that 

 as primrose and cowslip are different in appearance (not 

 to mention odour, habitat and range), and as I can now 

 show that, when they cross, the intermediate offspring are 

 sterile like ordinary hybrids, they must be called as good 

 species as a man and a gorilla. . . . The power of re- 

 maining for a good long period constant I look at as 

 the essence of a species, combined with an appreciable 

 amount of difference.' 2 



Introduction to the Discussion ' What is a Species?' 



The preceding pages have been occupied with pre- 

 liminary considerations necessary for a discussion of 

 the problem stated in the title of this Address. It is now 

 proposed to preface that discussion with a few words 



1 Neither Professor Lankester nor I have been able at present to lay 

 our hands upon the communication. 



2 More Letters, vol. i, p. 252, Letter 179. 



