( 23) 
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 pxove, in fact, hypothesis. 
It is evident too that Darwin regarded persistence of form 
as an important criterion of aspecies. We recognise this in the 
definition I have quoted from the “Origin” (see p. 10), and 
it is stated with even greater force in the following passage, 
where persistence is placed beside other distinguishing marks 
of aspecies 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 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 
aman anda gorilla. The power of remaining for a good long 
period 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 
oy Ska) dip dale Dixey i in ‘‘ Nature,” June 19, 1902, p. 169. 
+ ‘‘More Letters,” vol. i, p. 252, Letter 179. 
