INTRODUCTION. XXVl 
Now it is clear that from an investigation of all three 
classes just mentioned, we shall be able to gain an 
idea of those points which are common to all parts, to 
all individuals or to all aggregates, and those that are 
pecuhar to some of them, and, by eliminating the one 
from the other, we shall arrive at conclusions which 
will be more or less generally accurate or applicable, 
according to the ability of the student and the extent 
to which the comparative analysis is carried. Itis thus 
that morphologists have been enabled to frame types or 
standards of reference, and systematists to collocate the 
organisms they deal with into groups. These standards 
and groups are more or less artificial (none can be 
entirely natural) in proportion to the amount of know- 
ledge possessed by their framers, and the use they 
make of it. 
From this point of view teratological metamorphosis 
of all three kinds demands as much attention as that 
which is called normal. We can have no thorough 
knowledge of an organ, of an individual which is an 
ageregate of organs, or of an aggregate of individuals 
of whatever degree, unless we know approximately, at 
least, what are the limits of each. It is not possible 
to trace these limits accurately in the case of natural 
science, but the larger our knowledge and the wider 
our generalisations, the closer will be our approach 
to the truth. 
The most satisfactory classification of malformations 
would be one founded upon the nature of the causes 
inducing the several changes. Thus, in all organised 
beings, there is a process of growth, mere increase in 
bulk as it were, and a process of evolution or metamor- 
