CHAP. VIII 



Vitality 139 



through the surface, there must be a certain size of cell at 

 which the rate of absorption is just sufficient for the nourish- 

 ment of the protoplasm. Beyond this point a cell cannot 

 grow ; but if it divides, then the mass to be fed remains the 

 same, while the absorbing surface is increased. This, then, 

 is the necessitating cause of cell-division. But it would be 

 unwise to suppose that there are not other causes that help 

 to produce this result, which has as a consequence the 

 possibility of immense variety of disposition of the daughter 

 cells, and therefore of organic forms ; for, to begin with, a 

 more obvious means of obtaining increased surface would 

 be for the cell merely to become flattened or to spread out 

 irregularly, which, indeed, we see in many of the Protozoa. 



Since their growth implies cell-division as one of its con- 

 sequences, and since cell-division is the basis of reproduc- 

 tion, synonymous, indeed, in the Protozoa with reproduc- 

 tion, we get the idea of successive generations of animals as 

 merely the continued growth of former generations. This 

 makes intelligible to us all the facts of heredity which are 

 so surprising if we conceive of each generation as a number 

 of untried souls that have left some former dwelling-place to 

 come and live among us. Our children are, in truth, abso- 

 lutely portions of ourselves. If this be so, we must imagine 

 in the ovum the tiny mass of protoplasm from which we 

 are formed by continued division a most extraordinary 

 subtlety of constitution. 



Try to picture the complexity of the arrangement of parts. 

 There are two tiny masses of protoplasm ; so far as we can 

 see they are the same, yet from one will grow a man, from 

 the other a tree. If the germ that will grow into a human 

 being could only properly be fed outside the body of the 

 mother, so far as we know it might leave that body as 

 an almost invisible cell, and would grow and divide, add 

 cell to cell, until the creature was fully formed sculp- 

 tured out of dust and air. Our early life within the womb, 

 our nourishment by the blood of our mother, is only nature's 

 way of preserving us from injury. What we shall be is 

 already marked out before the egg begins to grow.- 



It is only the highest animals who are thus shielded. 



