Page:George Archdall Reid 1896 The present evolution of man.djvu/51

Rh reproduction, accordingly as they were best placed to perform it.

Now, since it cannot be doubted that low organisms vary as well as high organisms, we may legitimately suppose that some such variations as the above did occur, indeed we have abundant evidence that they must have occurred, and that they were so seized upon and accumulated by the action of natural selection (i.e. by the survival of the fittest), that such differentiations were thereby brought about in the forms of multicellular organisms (i.e. in masses of cells adherent for the common benefit), and such differentiations in structure and specializations in function in their component cells as resulted, after long ages and innumerable generations, in all the varied and wonderful forms of plant and animal life; and in the equally wonderful and varied differentiations in structures and specializations in functions of the cells composing those plants and animals; in such lordly- forms as the Wellingtonia Gigantia and the elephant, as well as in such lowly forms as the lichen and the hydra; in such highly differentiated cells as nerve muscle or gland cells, as well as in the white blood corpuscle, which may be likened to an amœba, or the bone cell, which secretes round itself a calcareous envelope like a rhizopod.

The single cell of the amoeba performs of necessity all the functions of life; but even in such low organisms as sponges a great amount of cell-specialization is already observable. In them cells which are differently situated as regards the environment differ somewhat in structure and function. All the cells are to some extent capable of performing all the functions of life, but some cells perform some one function better and other functions less well than other cells differently situated, which in turn display a like peculiarity. Thus as regards the function of reproduction some cells subserve