Hyatt.] 90 
[April s, 
period of full maturity is attained. Tiien as senility sets in and 
certain parts are lost or become withered, pronounced similarities 
again begin to a})pear. Thus in old age, especially in extreme 
old age, these same individuals are more closely i)arallel in aspect 
and character than during their adult periods. 
The same law holds in the evolution of the genus, family, or 
group, and the result is, as I have tried to point out in several 
jtapers, that retrogressive forms occurring in genera, or families, 
or larger groups of the same type, are apt to have similar forms 
in their retrograde or terminal members, wliich occur Avhen the 
type is appioaching extinction. Such genetic series, therefore, 
usually differ most when they are at the acme of their evolution 
in time. Thus, like different individuals of the same series, they 
are more alike just before they cease to exist than at the height 
of their existence in time. 
One can judge of the power of genism only by the more or less 
exact parallelisms produced. This is the law by which the em- 
bryologist proceeds when he successfully uses the stages of devel- 
opment in a young animal to indicate from what ancestors it 
came and in what group he must place it, and it is this law 
to which the breeder ajjpeals when he points out the mai'kings 
which alwaj^s occur in the pure-blooded specimens of any special 
stock. Consequently, if parallelisms are more exact in old age 
than in adults, these results must be attributed to the only cause 
that is known to produce such effects, and this is genetic force. 
I am not discussing here what causes senile de'cline and degen- 
eration in the parts of an individual and the consequent loss of 
characteristics, but I am trying to reply to the questions why, 
when this loss takes place, the oldest stages of different indi- 
viduals of a species and the corresponding phylogerontic types of 
different groups, arising from the same common ancestor, resem- 
ble each other. It seems to me to be in large part, at any rate, 
attributable to genism, Avhich holds the development of the indi- 
vidual and the evolution of the phylum true to the type even 
when growth is losing the power to continue the existence of 
individuals. 
Thus the evolution of forms in a phylum, or phylogeny, presents 
similar parallelisms to those observed in the development of the 
individual, or ontogeny. This shows still more clearly that 
lieredity is distinct from growth since the individual and the type 
