594 LAWS OF ORGANIC DEVELOPMENT. 
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 species, whose ori- 
gin has not been observed, have also descended from common pa- 
rents. So far I believe his induction to be justified ; but when 
from this basis evolution of divisions defined by important struc- 
tural characters, as genera, orders, classes, etc., is inferred, I be- 
lieve that we do not know enough of the uniformity of nature’s 
processes in the premises to enable us to regard this kind of proof 
as conclusive. i 
I therefore appeal to another mode of proving it, and one which’ 
covers the case of all the more really structural features of ani- 
mals 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 
a true identity of these adults with the various stages of the 
higher has, comparatively, rarely been observed. Let such a case 
be supposed. é 
In A we have four species whose growth attains a given point, 
a certain number of stages having been passed prior to its termi- 
nation or maturity. In B we have another series of four (the 
numbering a matter of no importance), which, during the period 
of growth, cannot be distinguished by any common, i. é.,, 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, and therefore held to have had a common origin or 
parentage (or still better the individuals of a single brood), which 
present differences among themselves of the character in question, 
we have gained a point. We know in this case that the individu- 
als, a, have attained to the completeness of character presented 
by group A, while others, b, 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 b. 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 develop- 
