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chance of difference between parent and offspring in highly- 
developed than in lowly-developed organisms. It may also be 
remarked that great change of conditions—as for instance, 
cultivation—causes considerable diversity among the descen- 
dants of a common ancestor. 
VARIATION. 
I wish to define Variation in a very restricted manner, and 
to confine the term to the difference between parent and 
offspring, when exactly the same age of each is compared. 
3 [Many phenomena are roughly classed as Variation, and it 
_ becomes necessary to make adistinction. Usually it is regarded 
as a departure from a normal standard, or from a given specific 
_ type; but this idea is entirely unsuitable when a phylogenetic 
series is under review. Paleontology illustrates a wonderful 
_ inconstancy among species. It impresses one with a sense of - 
_ constant change on the whole—a change slower in some groups 
_ than in others—and slower at some times than at others in 
_ the same genetic series. In a quickly developing group like 
_ Ammonites anything like the same form can only have endured 
for comparatively few generations, except in the case of the 
_ radical stocks. 
Practically speaking there was no normal type—practically 
speaking there was change in every generation. The same 
may be supposed to be theoretically true in slow-developing 
_ series—only the changes may have been and may be so slight 
as to be hardly perceptible. 
4 Little or great, it is these differences between parent and 
offspring at the same age which I wish to define as Variation. 
Why I insist on the same age is that the youthful offspring 
may differ very considerably from the adult parent, but such 
_unlikeness is not Variation, as now defined. In the course of 
its ontogenetical development an individual may change very 
considerably, passing through the same cycles of changes as its 
parent did. Such changes may be distinguished, not as 
“variations due to age” as is sometimes done, but as ontogenetic 
