no MANUAL OF ELEMENTARY ZOOLOGY 



This probability gives a peculiar interest to the fact, which 

 we have already noted, that as regards the nucleus ovum 

 and spermatozoon make equal contributions to the embryo. 

 Thereby, it would appear, the parents make equal contri- 

 butions to most, if not to all, of the inheritance of the 

 offspring. As to the way in which these contributions 

 realize themselves when they are of opposite tendency — 

 how they sometimes blend and sometimes one dominates 

 over the other so that the offspring "takes after" one 

 parent — we shall have something to say in later chapters 

 (pp. 197 and 527). 



Sooner or later, to frogs as to men, comes death. 1 

 . However successful the individual may be in 



avoiding enemies and accidents, he cannot 

 escape that gradual slowing of the working of the bodily 

 machine which in the long-run brings it to a standstill. 

 The course of metabolism is in some way limited, so that 

 in the most perfect conditions natural death would 

 eventually result. The nature of this limitation is not 

 understood. The fact that the germs are not subject to it, 

 but produce new individuals with a fresh lease of life, shows 

 that it is not inherent in all protoplasm, but belongs only 

 to that of the cells which constitute the bulk of the body. 

 This it would seem to affect in all tissues alike, so that, 

 were the body not the complicated machine that it is, 

 death might come as a gradual loss of power in all parts 

 alike. As it is, however, the end is always more or less 

 premature as regards some tissues, being brought about by 

 the breaking down of one of the main parts of the machine, 

 as the brain, or lungs, or heart, though in the long-run any 

 other part brings about general death through its effect 

 upon the heart, whereby the rest of the body is deprived of 

 fresh blood. It may be that this breakdown is due to the 

 protoplasm of the body cells being, as it were, wound up 

 to go for a certain length of time only. It may be that 

 some or all of them produce substances which are slow 

 poisons, and that these in course of time accumulate beyond 

 the power of the body to destroy or excrete them. In any 

 case, so far as our present knowledge goes, death in all 

 the higher animals is inevitable. 



1 It is said that frogs have been known to live twelve years. 



