Definition of a Species 
a distinct race is produced. And, finally, fresh 
mutations occur, so that a new species is eventu- 
ally produced. 
What De Vries calls an elementary species 
the majority of systematists would call a well- 
marked variety. 
We may take this opportunity of remarking 
that the definition of a species is one on which 
naturalists seem unable to agree. 
So vast is the field of biology, that now-a-days 
biologists are compelled to specialise to some 
extent. Thus we have botanists, ornithologists, 
those who devote themselves to the study of 
mammals, those who confine themselves to 
reptiles, or insects, or fishes, or crustaceans, or 
bacteria, etc. 
Now each class of systematists has its own 
particular criterion of what constitutes a species. 
Ornithologists do not seem very exacting. Most 
of them appear to consider a constant difference 
of colour sufficient for the formation into a species 
of the birds that display such a variation. Those 
who study reptiles, on the other hand, do not 
allow that a mere difference in colour is sufficient 
to promote its possessor to specific rank. Into 
these nice questions we cannot enter. For our 
purpose a species is a group of individuals that 
differ from all other individuals in displaying 
certain well-marked and tolerably constant charac- 
ters, which they transmit to their offspring. 
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