In the majority even of these _Protozoa_, the typical structure of the nucleated cell is retained, and the whole animal is the equivalent of a histological unit of one of the higher organisms. An _Amœba_ is strictly comparable, morphologically, to one of the corpuscles of the blood of the crayfish. Thus, to exactly the same extent as it is legitimate to represent all the crayfishes as modifications of the common astacine plan, it is legitimate to represent all the multicellular animals as modifications of the gastrula, and the gastrula itself as a peculiarly disposed aggregate of cells; while the _Protozoa_ are such cells either isolated, or otherwise aggregated. It is easy to demonstrate that all plants are either cell aggregates, or simple cells; and as it is impossible to draw any precise line of demarcation, either physiological or morphological, between the simplest plants, and the simplest of the _Protozoa_, it follows that all forms of life are morphologically related to one another; and that in whatever sense we say that the English and the Californian crayfish are allied, in the same sense, though not to the same degree, must we admit that all living things {286} are allied. Given one of those protoplasmic bodies, of which we are unable to say certainly whether it is animal or plant, and endow it with such inherent capacities of self-modification as are manifested daily under our eyes by developing ova, and we have a sufficient reason for the existence of any plant, or of any animal. This is the great result of comparative morphology; and it is carefully to be noted that this result is not a speculation, but a generalisation. The truths of anatomy and of embryology are generalised statements of facts of experience; the question whether an animal is more or less like another in its structure and in its development, or not, is capable of being tested by observation; the doctrine of the unity of organisation of plants and animals is simply a mode of stating the conclusions drawn from experience. But, if it is a just mode of stating these conclusions, then it is undoubtedly conceivable that all plants and all animals may have been evolved from a common physical basis of life, by processes similar to those which we every day see at work in the evolution of individual animals and plants from that foundation. That which is conceivable, however, is by no means necessarily true; and no amount of purely morphological evidence can suffice to prove that the forms of life have come into existence in one way rather than another.