ORGANIC EVOLUTION — PHYSICAL 39 



reproduction, accordingly as they were best placed to 

 perform it. 



Now, since it cannot be doubted tliat 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. c. 

 by the survival of the fittest), that such differentiations 

 were thereby brought about in the forms of multicellu- 

 lar 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 gener- 

 ations, 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 amoeba, 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 organ- 

 isms 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 ex- 

 tent 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 



