One of the difficulties attending the study of "species" among the
higher forms of plants and animals has always been the length of time
required to obtain any large number of generations on which to make
observations. In the case of such plants as peas, wheat, corn, or indeed
almost any form of plant life, it is only with difficulty that more than
one generation a year can be obtained; and when two or more generations
a year are produced, they are produced under more or less unnatural
conditions. So that it takes almost a lifetime carefully to test and
record in a thoroughly scientific way the results of any extensive
experiments regarding variation and heredity.
In the case of mice or rats or rabbits or guinea pigs, many more
generations can be obtained in a few years; but in the case of the
larger kinds of animals the time taken for development to maturity and
for gestation is often much prolonged; and scientific observation of an
exact character has been in vogue for so short a time that there has
always been the chance for advocates of evolution to take refuge under
the plea that, if we only had longer and more carefully conducted
observations, we could really see species in the making, one form
becoming transformed into a distinct form, or perhaps giving rise to
another and distinct form as an offshoot.
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