No. 381.] THE CONCEPTION OF SPECIES. 691 
our eyes to see the value of evolution in general, the majority 
of the active botanists of the present day find too many other 
pressing questions to be solved to be able to dwell on evolution 
to the same exclusive extent as did the botanists of the last 
generation. 
Our definition of a species included two terms which required 
further explanation. We started out in the hope of finding 
some light as to the approximate length, or at least the approx- 
imate minimum of the length of time which is needed to trans- 
form a race into a species, hoping that perhaps those plants in 
which the development of the individual was rapid might show 
that in a comparatively short space of time a race might be 
actually observed to become fixed and be considered a species ; 
a fact which certainly could not be so well ascertained by 
direct observation in the study of the higher plants alone. You 
will notice that, like the obliging shopkeeper, I have not given 
you exactly what you expected, but have offered you instead 
something else perhaps just as good, if not better. If I have 
not been able to tell you that in such simple and quickly grow- 
ing plants as bacteria and Saccharomycetes new species can 
be produced from old ones in a comparatively short time, a con- 
sideration of some of the peculiarities of such plants has brought 
out the modifications which have taken place in the views of a 
good many-as to specific limitations, which is in part an answer 
to our primary question, What do we mean by a species ? 
It may be added that although some of the species of lower 
plants may be transformed in various ways by artificial cultures, 
on the whole we are quite as much struck by their comparative 
constancy in important respects as by their tendency to differ- 
entiate. In Uredinacez there is a tendency to form adaptive 
races, which is greater than was formerly supposed; but whether 
the tendency is greater than would be found in some higher 
plants, were they studied as carefully as have been the Uredi- 
nacez, is perhaps a question. Parasites, as a rule, are more © 
plastic and more sensitive to changes of environment than other 
plants, and their impressionability, if I may use that word, might 
be expected to accentuate their power of specific transformation. 
It cannot be denied that there is a general suspicion, to say knowl- 
