THE HISTORY OF ANATOMICAL SCIENCE 325 
In trying to form a judgment of the value 
of the explanation offered, it is a necessary 
preliminary to consider what there was to ex- 
plain. 
Among the animals with which we are familiar, 
proliferation, or the production of offspring, inva- 
riably implies the concurrence of two parents, a 
father and a mother. We are, therefore, naturally 
led to regard this method of proliferation as the 
rule, and any other as an exception. But, as we 
have seen, if our daily experience had been 
derived from many of the lower animals and 
plants, we might just as well have been led to 
think sexless proliferation the rule, and the 
other the exception. Whatever the outward 
form of the process of proliferation, in sub- 
stance it always comes to the same thing. It 
is the detachment of a parcel, A, of the living 
substance of the parent, which either before, or 
after, detachment evolves into a complete, physio- 
logically independent, organism. There are in- 
numerable cases in which this process takes place, 
in virtue of the autonomous activities of the living 
substance of an organism. The progeny in this 
case is a detached fragment of A, and nothing 
else. Why is it that, in equally numerous other 
cases, a parcel of the same kind may be similarly 
detached from A, but does not evolve, unless an- 
other parcel, B, of living substance, derived from 
the same, or another, organism, not merely comes 
