into some cells, leaving the other cells without it.  But in any case, after this process of cell-division has proceeded for a certain time, differentiation begins to set in—­some cells become modified in one way, others in another, and from what was a relatively homogeneous mass an organized embryo, with highly differentiated parts, appears.  The problem immediately propounds itself—­what are the factors which control this differentiation?  This problem is essentially a physiological one, and yet, since it arises most conspicuously in a field which has been worked by professed zoologists rather than physiologists, it has been studied more by those trained in zoology and botany than by those who have specialized in physiology.  In this way, as in many other directions, such as in the study of heredity, of sex, and of the effects of the environment on the colours and structure of animals, the trend of zoology in recent years has returned towards the physiological side, and the old division which separated the sciences (but which has never so seriously affected students of plant life) is being obliterated.

Hence we are led back to consider the progress of Physiology as a whole—­a subject with which the present writer hesitates to deal except in a very superficial manner.  Physiology as an organized science has inevitably been deeply influenced by its close relation with medicine, with the result that through a large portion of the period under review it has concerned itself chiefly with the functions of the human body in particular, or at least chiefly with Vertebrates from which, by analogy, the human functions may be inferred.  In this field it has made enormous progress, and a vast amount of knowledge has been gained with regard to the function and mechanism of all the parts and organs of the body.  It may perhaps be suggested, however, that in the pursuit of this detailed (and in practice absolutely necessary) knowledge, physiologists have to some extent lost sight of the wood in their preoccupation with the trees.  That is to say, while they have advanced an immense distance in their knowledge of organs, they have not yet got as far as might be hoped in the understanding of the organism—­which is to say no more than that the great and fundamental problem of Biology, the nature and meaning of Life, is apparently almost as far from solution as ever.  To this further reference will be made below.

The progress of Physiology has been so great in all its branches that it is difficult to decide which most deserve mention; perhaps the most important advances are those connected with the nervous system and with internal secretions.  Little or nothing was known fifty years ago of the minute structure of the nervous system, nor of the special functions of its different parts.  Now the main functions of the various parts of the brain, and the relation of these parts to the activities of the other organs of the body, are well known, although much remains to be discovered with regard to the more detailed localization of function.  The study of the microscopic structure of brain and nerve, and experiment on the conduction of nervous impulse, have given us some insight into the mechanism of the nervous system, but the fundamental nature of nervous action still remains unsolved.