of evolution, one has, therefore, to examine living fauna, not only as regards the resemblances, differences, physical environments, and geographical distribution of the different species, but also to trace their fossil predecessors throughout a long series of geological periods extending from the dawn of life on the globe to the present day. When a common fossil ancestor to two or more present-day species is disinterred, we have to carry our imagination back to the time when it was simply an individual member of a living species, similarly affiliated to pre-existing predecessors. By continuing this line of investigation backwards in time through the successive sedimentary strata which form the larger portion of the earth's crust, we gradually encounter less and less specialised forms, till ultimately the most divergent types meet in a common origin among unicellular organisms, analogous to those still prevalent on the borderland of the animal and vegetable kingdoms, such as the amœba, red-snow plant, yeast, etc. Now many of these lower organisms exhibit, though in a most rudimentary manner, nearly all the vital phenomena of the higher animals—voluntary motion, assimilation of food, growth and maintenance, reproduction, decay, and death.
It has been demonstrated by the dissection of animals that, however dissimilar in outward form, they are all a congeries of unicellular organisms grouped together on a uniform plan, and working in the animal economy on the principle of the division of labour. Hence the human body may be regarded as a combination of unicellular animals, arranged so as to subordinate their original individualism to the general welfare of the compound organism. In the course of long ages these cell-units have become adapted for their special functions by becoming part of the animal's tissues and framework, or are floating about as corpuscles in its blood and secretions. Thus they build up cellular membrane, muscular fibre, medullary matter, brain cells, etc., by means of which the animal functions, from simple molecular movements up to conscious cerebration, are performed. It may also be observed that, between the unicellular organisms and the higher animals existing in the present day, there are multitudes of intermediate groups more or less differentiated; and so gradual is this differentiation
