14 _ LIVERPOOL BIOLOGICAL SOCIETY. 
All these groupings of individuals have been produced 
as the result of evolution by the action of natural selection 
(along with, it may be, some supplementary secondary 
factors such as sexual selection, use and disuse, direct 
action of the environment and physiological selection), and 
consequently I believe that their characters will be found 
to be such as could be seized hold of and perpetuated by 
the action of natural selection. But great care must be 
taken in this inquiry that the true generic and specific 
characters are dealt with. It often happens in systematic 
work that we find a particular generic or say specific 
character, or combination of characters, to be useless, i.e. 
not really diagnostic of the species. It is perhaps common 
to many species, or it may be is not of constant occurrence. 
It becomes necessary then to change our ideas in regard 
to that species, to re-define it cr even to abolish it. Such 
necessary changes in the recognized specific characters - 
frequently take place during the revision of a group. 
Consequently, in the proposed inquiry into the usefulness 
or the reverse of specific characters it will not do merely 
to take the specific characters now published in the books 
on the subject. It will be necessary to subject the species 
in question to a rigorous examination and to re-determine 
their specific characters; and this while adding to the 
labour will add greatly to the value and interest of the 
work. 
But it will not be sufficient to examine the adult 
animals alone. In some groups at least it will be neces- 
sary to investigate the whole life-history, and to study 
especially those stages when the young animal leads an 
independant life. We know that in many such cases large 
and remarkable larval structures are formed which are 
of very great importance to the possessor during a brief 
period of its existence, and are then usually got rid of in 
