al 
1871.] 231 
I. ON THE LAW OF ACCELERATION AND RETARDATION. 
There are two modes of demonstration of evolution, both depending on 
direct observation. One of these has been successfully presented by 
Darwin. He has observed the origin of varieties in animals and plants, 
either in the domesticated or wild states, and has shown, what had been 
known to many, the lack of distinction in the grades of difference which 
separate varieties and species. But he has also pointed out that species 
(such, so far, as distinctness goes) have been derived from other species 
among domesticated animals, and he infers by induction that other spe- 
cies, whose origin has not been observed, have also descended from com- 
mon parents. So far, I believe his induction to be justified ; but when 
from this basis evolution of divisions defined by important structural 
characters, as genera, orders, classes, etc., is inferred, I believe that we 
do not know enough of the uniformity of nature’s processes in the prem- 
ises to enable us to regard this kind of proof as conclusive. 
J therefore appeal to another mode of proving it, and one which covers 
the case of all the more really structural features of animals and plants. 
It is well known that in both kingdoms, in a general way, the young 
stages of the more perfect types are represented or imitated with more or 
less exactitude by the adults of inferior ones. But atrue identity of these 
adults with the various stages of the higher has, comparatively, rarely 
been observed. Let such a case be supposed. 
In A* we have four species whose growth attains a given point, a certain 
number of stages having been passed prior to its termination or maturity. 
In B we have another series of four (the number a matter of no import- 
ance), which, during the period of growth, cannot be distinguished by 
any common, 7. ¢., generic character, from the individuals of group A, 
but whose growth has only attained to a point short of that reached by 
those of group A at maturity. Here we have a parallelism, but no true 
evidence of descent. But if we now find a set of individuals belonging to 
one species, or still better, the individuals of a single brood, and therefore 
held to have had a common origin or parentage, which present differences 
among themselves of the character in question, we have gained a point. 
We know in this case that the individuals, @, have attained to the com- 
pleteness of character presented by group A, while others, }, of the same 
parentage have only attained to the structure of those of group B. It is 
perfectly obvious that the individuals of the first part of the family have 
grown further, and, therefore, in one sense faster, than those of group 0. 
If the parents were like the individuals of the more completely grown, 
then the offspring which did not attain that completeness may be said to 
have been retarded in their development. If, on the other hand, the 
parents were like those less fully grown, then the offspring which have 
added something, have been accelerated in their development. 
I claim that a consideration of the uniformity of nature’s processes, or 
inductive reasoning, requires me (however it may affect the minds of 
others) to believe that the groups of species, whose individuals I have 
*A cut explaining this proportion will be found at the end of the essay. 
