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Transactions of the Society . 
Lave no right to maintain that the sterility of species when first crossed, 
and of their hybrid offspring, is determined by some cause funda- 
mentally different from that which determines the sterility of the 
individuals, both of ordinary and heterostyled plants, when united 
in various ways.” * Even if there be no difference, it does not affect 
the point at present under consideration. As previously observed, 
we have to do, not with the cause of the sterility, which may 
eventually prove to be completely the same, but with its effect, which 
is certainly quite different. In the case of the comparatively few 
plants which have become heterostyled, illegitimate unions must 
seldom occur ; and the effect of their sterility is neutralised by the 
complete fertility of the legitimate unions, for the consummation of 
which, by means of insect agency, special facilities are provided. 
There is, therefore, no cessation of interbreeding, and no consequent 
disappearance of intermediate forms. In the case of the far greater 
number of plants which have not acquired the highly specialised 
condition of the heterostyled, there does ensue the hindrance te 
interbreeding, and thence the breach of continuity already described. 
It is this interruption of the series of intermediate forms, as they 
now exist, which appears to me to supply the practical means of 
discrimination between the species of the present time. It is 
obviously necessary to thus confine our attention to contemporary 
forms. If we do otherwise, and bring into view fossil forms which 
formerly existed, it is evident that what are now distinct species, 
incapable of intercrossing, and therefore with a gap between, might 
be found, at a comparatively recent geological period, to graduate 
into one another. Indeed, but for the imperfection of the geological 
record, every existing species of whole orders and classes would be 
found connected by complete series of transitional forms with their 
common remote ancestor. 
This view of species appears to receive some degree of support 
even from the late Mr. Romanes ; for in the second part, published 
after his death, of his ‘ Exposition of the Darwinian Theory,’ among 
his “ logically possible definitions of a species,” the second runs as 
follows : t “ A group of individuals, which, while fully fertile inter se y 
are sterile with all other individuals — or at any rate do not generate 
fully fertile hybrids.” He remarks that “ This purely physiological 
definition is not nowadays entertained by any naturalists ” ; but 
he afterwards J observes, “ As species have been actually constituted 
by systematists, the test of exclusive fertility does not apply. For 
my own part I think this to be regretted, because I believe that such 
is the only natural — and therefore the only firm — basis on which 
specific distinctions can be reared. But, as previously observed, this 
is not the view which has been taken by species-makers.” 
The non-adoption by systematists of “ exclusive fertility ” as the 
* ‘ Cross- and Self-Fertilisation of Plants/ p. 466. 
f ‘ Darwin, and after Darwin/ part ii. p. 229. X Tom. cit., p. 236v 
