The notion that all the kinds of animals and plants may have come into
existence by the growth and modification of primordial germs is as old as
speculative thought; but the modern scientific form of the doctrine can be
traced historically to the influence of several converging lines of
philosophical speculation and of physical observation, none of which go
farther back than the seventeenth century. These are:--
1. The enunciation by Descartes of the conception that the physical
universe, whether living or not living, is a mechanism, and that, as such,
it is explicable on physical principles.
2. The observation of the gradations of structure, from extreme simplicity
to very great complexity, presented by living things, and of the relation
of these graduated forms to one another.
3. The observation of the existence of an analogy between the series of
gradations presented by the species which compose any great group of
animals or plants, and the series of embryonic conditions of the highest
members of that group.
4. The observation that large groups of species of widely different habits
present the same fundamental plan of structure; and that parts of the same
animal or plant, the functions of which are very different, likewise
exhibit modifications of a common plan.
5. The observation of the existence of structures, in a rudimentary and
apparently useless condition, in one species of a group, which are fully
developed and have definite functions in other species of the same group.
6. The observation of the effects of varying conditions in modifying living
organisms.