MERRIAM — SPECIES AND GENERA 
7 
from the theoretical standpoint these methods are as far apart as pos- 
sible, in practice they draw much nearer together. For in the great 
majority of cases of alleged intergradation the intergradation is assumed 
rather than proved, so that after all the student is influenced, albeit 
unconsciously, by the quantity of difference — this being in reality the 
determining factor in shaping his decision as to whether or not inter- 
gradation exists. 
But in studying animals and plants, what difference does it make, 
(a) whether the worker has before him actual intergrades, or (b) 
whether the relationship between forms is so close that he feels justi- 
fied in assuming intergradation, or (c) whether in fact at the present 
moment of the world’s history intergradation does or does not exist? 
For is it not clear that in the course of evolution, intergrades, if not 
now present, must have existed in the past, so that their remains are 
likely to turn up at any moment? And is it not equally clear that if 
we are to destroy species and genera because of the presence of inter- 
grades, it is only a matter of time before the discovery of living forms 
or the accumulation of paleontological evidence will lead to the aboli- 
tion of a large proportion of our species and genera? 
To my mind, the criterion of intergradation is one of the most per- 
nicious that has ever been introduced into the systematic study of 
animals and plants and one necessarily productive of an ever-changing 
nomenclature. And furthermore, it has often resulted in bringing 
together forms between which intergradation has not only not been 
proved, but which in many cases never existed — the forms in question 
having arisen from a common ancestry in the distant past, rather than 
from one another under existing conditions. And even in the case of 
forms presumably derived from one another under existing conditions, 
what difference does it make whether the specimens at hand prove 
intergradation, or whether the closeness of their interrelationship 
implies that one is an offshoot of the other? Is not the measure of rela- 
tionship of more consequence than the accident of survival or non-sur- 
vival of intergrades? 
In practice, neither the criterion of intergradation nor that of degree 
of divergence can claim immunity from the mistakes that come from 
the study of inadequate material, nor from those due to the idiosyn- 
crasies of the personal equation, for until the arrival of the biological 
millennium personal opinion is likely to govern the existence or non- 
existence of intergrades and to stand in the way of agreement as to 
the measure of difference necessary to the recognition of species and 
genera. 
