1894. | NEW YORK ACADEMY OF SCIENCES. 133 
a 
prominence it deserves in the study of the complex phenomena 
which we know under the general name of variation. 
The life history of almost any plant one takes up illustrates 
atavism in its vegetative organs; this is perhaps most readily 
observed in the first-developing leaves of Dicotyledons. Take 
the poplars and aspen for example (Populus). In these the 
leaves of young trees, and to a less marked degree the first-ap- 
pearing ones of older trees, are radically different from those of 
subsequent appearance. The same is true of many of the oaks. 
In the Australian genus Hucalyptus the leaves of the lower and 
therefore earlier branches are dissimilar to those of the upper in 
at least several species. The North American Tulip-tree (Lirio- 
dendron) occasionally shows leaf-forms very different from the 
normal, and these were recognized by Dr. Newberry as charac- 
teristic of Cretaceous ancestral ‘ species.” Doubtless all these 
phenomena are susceptable of the same explanation. 
Examples of such occurrences might be almost indefinitely 
multiplied. From this series of facts we are led to suspect that 
many divergences from what we recognize as the most usual 
characters of an organism may be merely some features of its 
ancestors, perhaps of some exceedingly remote generation, 
brought out again by this most wonderful tendency, sometimes 
to be transmitted to its immediate offspring, sometimes to perish 
with it, sometimes to appear again only after centuries. Now, 
inasmuch as we understand that the groups of similar organisms 
which we term species, inhabiting the same general area of the 
earth, have descended from common ancestors, we must expect 
that any individual or generation of them will be susceptible to 
this tendency, and may exhibit characters which will ally them 
more closely than usual with their relatives of what we com- 
monly understand to be another species. When this happens 
to be observed, and it is my belief that it is, perhaps unwittingly, 
very frequently observed, it has generally been disposed of by 
concluding that the two organisms hitherto regarded as different 
are really but forms of the same thing, or that the two “ species ” 
are one, notwithstanding the fact that the great bulk of indi- 
viduals forming the two groups may be entirely distinct, even 
to casual observation. Does it not appear, however, that this 
method of treatment is open to grave philosophical objection in 
the light of facts as set forth above? Is it not better to regard 
such cases as distinct species and recognize the occurrence of 
what the ornithologists have termed “ intergrades ?” 
While I have given atavism the place of honor in this discus- 
sion, I would not have it supposed that I maintain it to be the 
chief cause of variation in living organisms. I have taken it 
