542 
of individual development which would entitle us to infer 
that it has also been a factor in the evolution of the race. 
If all animals were born in a perfect form, except so far 
as growth and the development of the sexual elements 
were concerned, we might have some difficulty in drawing 
any inference on the subject; for it might be contended 
that the development which took place before birth occurred 
independently of external conditions, and was the result of 
heredity; and that it was therefore impossible to investigate 
the causes which brought about these changes in the first 
instance. Even if this were so, we might glean some hints 
upon the subject. In some instances, the young while yet 
unborn are fully equipped for an aquatic life, although 
destined when born for a terrestrial one. The Alpine 
salamander is born in the fully developed condition with 
out gills. In the uterus, it is provided with very long 
gill-filaments.* And the inference from this fact is, that 
the ancestors of the animal were gill-breathers before they 
became lung-breathers. 
But sometimes the process is very much curtailed, and 
it might be dangerous to draw our inferences too freely. 
But fortunately for our present enquiry, many animals are 
not born with the perfect forms which they subsequently 
acquire, and we are thus enabled to make these imperfect 
or larval forms the subject of observation and experiment; 
and the result of this investigation favours the idea that 
nature has exercised a transforming, as opposed to a 
selective, influence. It is interesting, at this point, to 
observe that the difference in the stage of development 
at the time of birth is determined by circumstances. 
“ The ringed snake lays eggs which require three weeks’ time to 
develope. But when it is kept in captivity, and no sand is strewn in 
Balfour, vol. it., p. 142.