530 REPORT—1883, 
taken—those which demonstrate the polymorphism of the lowest forms of 
animal and vegetable life to which the Schizomycetes are most nearly related, 
and those which indicate the polymorphism of Diseases accounted ‘ specific’ by the 
pathologist. In the pre-Darwinian days in which every species of plant or 
animal was regarded as a special creation, permanently transmitting its distinctive 
peculiarities from parent to offspring, there were two schools of botanists and 
zoologists: one laying the greatest stress on minute differences, and multiplying 
species to an extravagant extent; while the other, looking rather to points of 
agreement, to the gradational characters presented by the differences when the 
comparison is made between a sufficiently large number of forms collected from a 
wide area of distribution, and to the modifying influence of external conditions, 
aimed to reduce the number of specific types by making a large allowance for 
‘range of variation.’ Among the flowering plants of Britain, for example, nearly 
1,700 species were enumerated by ore distinguished botanist ; while another re- 
duced the number below 1,200—chiefly by the suppression (on the foregoing 
grounds) of the large number of species contained in the variable genera Rosa, 
Rubus, and Sakix. And whilst D’Orbigny, in his classification of Foraminifera, 
enormously multiplied Genera as well as Species, by selecting only the most 
strongly differentiated types, later systematists, by tracing out the gradational 
connections between these, have greatly reduced their number. 
The Evolutionist who looks at species simply as races, which, having come to be 
differentiated from each other, transmit their respective differentie by genetic 
descent, is prepared to admit any amount of such gradation; the supposed per- 
manence of specific types being, in his view, simply the result of persistence of the 
same external conditions, and giving place to change of type whenever these condi- 
tions undergo any essential modification. The well-informed botanist or zoologist, 
then, no longer entertains the idea of fixity in natural species; and specific 
designations are now only used provisionally, as indicating races in which well- 
marked differential characters have been found to be transmitted genetically so long 
as our term of observation has lasted. To the amount of change in any race which 
might take place under the influence of small variations in external conditions, 
acting persistently through a long succession of generations, no scientific Naturalist 
would now feel justified in assigning limits. 
The author desires to lay special stress upon two orders of facts, which he 
believes to be familiar to every experienced naturalist. It not unfrequently 
happens, in the first place, that two types of plants or animals present in one 
locality very well marked differential characters, and transmit these with such 
genetic continuity as apparently to justify the ranking them as distinct species; 
while yet, in some other locality, they are found to be connected by such a grada- 
tional series of intermediate forms that it is impossible to draw a definite line of 
demarcation between them. And, secondly, range of variation, arising out of 
the modifying influence of external conditions, shows itself more strongly in the 
lower than in the higher forms of vegetable and animal life; and in no group more 
strongly than in the simplest Fungi (the ‘moulds’ and ‘ blights’) to which the 
Schizomycetous ‘ disease-germs’ are most nearly related. From the natural-history 
point of view, therefore, we should expect that instead of always developing them- 
selves in one particular mode, and giving rise, by their introduction into the human 
body, to one fixed and constant type of morbid action, those different forms of 
bacilli, bacteria, or micrococci, which we are now learning to regard as the germs 
of the different species of zymotic disease recognised by the Pathologist, should be 
capable of undergoing considerable modification according to the conditions under 
which they are developed, especially when those conditions act persistently through 
a succession of generations. And we should further expect that,as the diseases 
which we now regard as specifically distinct have come to be so by a process of 
evolution, so, notwithstanding the well-marked differentise which they may usually 
present, they may in other localities or at other times graduate insensibly into each 
other—a mild disease-germ developing itself into a virulent one, or, conversely, a 
very severe type of disease showing itself under an extremely mitigated form. Now 
this is precisely what the culture-experiments of Pasteur and his followers have 
