36 EVOLUTION OF LIVING OKGANISMS. 



of substance and particular association of qualities is 

 transmitted by means of the reproductive cells ; hence 

 the importance of the principle of the continuity of living 

 protoplasm established above (p. 25). The direct con- 

 tinuity of living substance, obvious in the case of the 

 unicellular organisms which reproduce by fission, is no 

 less essential in all other modes of propagation. In this 

 way only can the physico-chemical and other properties 

 of one generation pass on to another. The process of 

 metabolic change has continued without interruption since 

 its first appearance from generation to generation. 



The full importance of heredity is perhaps, even yet, 

 not always appreciated. This or that striking peculiarity 

 reappearing in parent and child is pointed at as having 

 been inherited, as if the countless resemblances in the 

 whole organisation were not also dependent on heredity. 

 Not only the similarities, but all the vast differences 

 which distinguish man from the lowest animal have 

 been built up through heredity. Without it no evolution 

 could take place. For we must think of the continuous 

 stream of protoplasm, the physical basis of evolution, as 

 the means of piling up, so to speak, hereditary differences 

 along diverging lines. 



In ordinary sexual reproduction, the specific substance 

 containing the essential factors of inheritance must be 

 passed on from one generation to the next in the germ- 

 cells. It forms either the cell as a whole or only its 

 nucleus ; many authors indeed identify it with the chro- 

 matin. Moreover, it is carried in the germ -cells of both 

 sexes, since inheritance is equal from both parents. The 

 characters due to these factors usually reappear in the 

 same order in the offspring in which they developed in 

 the parent ; but some of them may remain for ever latent, 

 though capable of reappearing in later generations. 



Now the older writers on evolution generally assumed 

 that the course of heredity and the progress of evolution- 

 ary change were greatly influenced by the direct action of 



