"I should expect that the period of gestation will differ very little in the individuals of the same species, as long as its conditions of life remained the same. But I doubt whether it is sure as an absolute criteria; for although little or nothing on this field can be known with respect to species in a state of nature, yet with races of the same species as with dogs and cattle, the period is known slightly to differ. In the generation of seeds from the same capsule there is often the most wonderful and inexplicable difference in the periods".