AN UNNAMED FACTOR IN ORGANIC CONSTRUCTION. 125 
The science of evolution requires mucli more attention to be paid 
to the conditions of life, which must have had immense influence 
in developing the varied species as pointed out by Darwin. The 
species seemed to be formed by these conditions as by a mould. 
Were these conditions known to the old philosophers as the 
"mothers"? "Were they not the "environments" of Herbert 
Spencer ] If species were to a great extent formed by their habits, 
would not the term habitation be suitable 1 
In developing to fill more difficult conditions, or take up their 
abode in higher habitations (using the word in the sense above 
indicated), more seemed necessary than mere " survival of the 
fittest" as adapted by " irrelative spontaneous variation." For 
instance, if we take the evolution of the canine tooth as repre- 
senting a unit by which we might gauge the approximate time 
which it would take mere external conditions to evolve a particular 
type of tooth, the external conditions which it had to fulfil 
seem to be those requiring a weapon of offence or defence, con- 
ditions constant throughout nearly the whole range of animal life, 
both in time and space ; yet this tooth was not finally -pronounced 
until the appearance of mammals. Its development seems to 
have been from the lengthening of one or more teeth in the jaws of 
predatory animals as a death-dealing weapon, sometimes appearing 
in one position, sometimes in another, or several in the same jaw 
in different positions, as evidenced by several crocodiles, fishes 
(dentex), &c, as if feeling about for the best place for use, coinci- 
dent with a shortening in the jaw to give more power, until 
reaching its highest type in the lion or tiger in the front of a short 
jaw, and yet wide apart. 
A ruminant, on the other hand, has been developed by conditions 
much less extended both in time and space, and yet its teeth are of 
much greater complexity, are exceedingly well defined, and extend 
through a great many contiguous genera. Traces are to be found 
as if they arose among the pigs, and after becoming typical amongst 
the ruminants, died out among the horses and rhinoceri. 
The type of the ruminant grinding tooth, which is that of a 
double crescent, involving a complicated involution of dentine, 
enamel, and cementum, is presented in most of the species in all 
the twenty-four back teeth, and not only so, but the twelve lower 
teeth, in order the more perfectly to reciprocate in action against 
the upper, stand in the inverse position in the jaws. Now if we 
