46 HUMAN PHYSIOLOGY 



When we think of our ancestors who have died 

 perhaps fourteen of them in a hundred years we must 

 be impressed by the disproportion between the mass of 

 their tissues which perished and the infinitesimal survival 

 in ourselves of what they transmitted to us. Someone 

 has said that the body is like a great lantern, serving 

 primarily to save the tiny, trembling flame of the germi- 

 nal life from being extinguished. It is a lantern which 

 wears out and only the flame can be saved to burn for a 

 time in a fresh protective shell. One contrast between 

 single-celled and many-celled forms of life will now be 

 clear: in the former the whole substance of one genera- 

 tion may live to constitute the next, in the latter the 

 accumulated cells of the body die and under the most 

 favorable conditions leave but the minutest part to 

 represent the stock. 



Another difference between the unicellular and the 

 more highly developed organisms is associated with sex. 

 A single bacterium may give rise to two, and these to 

 four descendants, and so on. A solitary member of 

 any of the more advanced types must mate with another 

 of opposite sex if the species is to be reproduced. Tne 

 fertilized ovum, before mentioned, which develops into 

 the embryo and so into the mature individual may be 

 called one cell, but it is more truly a composite of two 

 half-cells, one furnished by the male and one by the 

 female. Thus the animal of the higher sort has two 

 parents, four grandparents, eight ancestors in the next 

 generation, the numbers soon becoming enormous as we 

 reckon backward. The bacterium, on the other hand, 

 has but one ancestor in a generation. 



The case of the one-celled animal forms is not so simple 

 as may have appeared from the unqualified statements 

 that have been made. Close observation of these 

 orders of life has shown that, while the multiplication 

 of cells by cleavage is the common process among them, 

 in many instances the fusion of two cells into one (con- 

 jugation) is a possibility. When it takes place there is 



