( X" ) 



and great advantages in thus making a fresh start and in 

 the abandonment of " species," or the restriction of the 

 word to the only meaning it originally possessed before 

 it was borrowed from logic to become a technical term in 

 zoology.* 



Professor Lankester in former years published (I cannot 

 at this moment lay my hands upon the communication) 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 be- 

 yond the late pre-historic times (or beyond any other arbi- 

 trary 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 con- 

 ception of species would very often mean a great deal more 

 than we can pi'ove, in fact, hypothesis. 



It is evident too that Darwin regarded persistence of form 

 as an important criterion of a species. We recognise this in the 

 definition I have quoted from the " Origin " (see p. Ixxviii), 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 to the death 

 that as primrose and cowslip are diiferent 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 remaining for a good long 

 pei'iod constant I look at as the essence of a species, combined 

 with an appreciable amount of difference." t 



It is now necessary to examine in some detail the most 

 usual conception of a species, a conception based upon 

 distinguishing structural characters, or diagnosis. 



This idea of a species is clearly expressed by Sir William 

 Thiselton Dyer, when he speaks of the older writers who 



* See F. A. Dixey in "Nature," June 19, 1902, p. 169. 

 t "More Letters^" vol. i, p. 252, Letter 179. 



