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He said, differentiation means the setting apart of different members 

 for different work, and specialization of function means their gradual 

 adaptation to their special work in such wise, that in physiology at 

 least they become unfit for any other purpose. He proceeded : — " Now, 

 with this idea in our minds, let us survey the world of life. Just as the 

 world of commerce stretches from the lowly barrows of the costcrmongers 

 to the towering palaces of the London merchants, and includes every 

 intermediate grade, so in like manner the world of life stretches from 

 the lowly yeast cell, or the zoophytes and protozoa, at one end, to man 

 at the other. It is of course the received belief in the world of science, 

 that in the world of life, through the course of countless ages, the higher 

 species have been evolved or developed from the lower, that there is no 

 breach of continuity any where, but that by gradual improvement this 

 mighty progress has been brought about. Of this I propose to say nothing 

 to-night, all I shall try to show is that the point wherein the higher 

 species differ from the lower, is in differentiation and specialization of 

 function. There is one further direction in which I shall trace this 

 progress, and that is in the life history of the higher animals, from the 

 embryo to the full formed organism. Tf, for example, we take the egg 

 of a fowl and watch the different stages through which it pusses 

 during the time of incubation until the full formed chick bursts the 

 shell, we shall see the same process goiug on. "What a wondrous process 

 it is, and yet it is one of differentiation and specialization of functions, 

 the gradual formation of separate organs, each fitted for its own work 

 and for no other. I will now illustrate this principle more in detail, 

 first, by surveying the world of life generally, and passing from the 

 lowest to the highest forms, to show how the highest differs from the 

 lowest, precisely in this higher degree of differentiation, and secondly, by 

 taking the life history of one single highly organized creature, and 

 showing you the different stages. 



It will be well to give a moment's thought to our own frame work ; 

 to realize more fully the specialization of function. There is one part of 

 us, and only one part of us, which can do the work of digestion ; it is no 

 use to put food in our nose. The eye, and the eye alone can see, and it 

 can do nothing else. The nerves, and the nerves alone can fee!, when we 

 have a toothache, the dentist kindly kills the nerve, and toothache in 

 that tooth at least is impossible. The lungs give oxygen to the blood ; they 

 have no other purpose whatsoever. To some slight extent the blood receives 

 some oxygen through the pores of the skin, but that is of little purpose 

 if the lungs do not work. The veins carry blood, and do nothing what- 

 ever else, so that we have a series of organs highly specialized, limited 

 each for its own peculiar purpose. Let us turn to the other end of the 

 scale. The simplest form in which we find life is what we call a cell. 

 This, if you will forgive me a simple illustration, is something like the 

 capsules in which we now give cod liver oil to children ; there is a sort 

 of gelatinous bag, and an apparently homogeneous substance inside, of 

 course microscropically small. The cell is in principle the same in low 



