CHAPTER VII. 



The Causation of Structure (continued) — Heredity — Sexual and Asexual Ke- 

 production — Keproduction by Simple Division — Parthenogenesis — Graft 

 Hybridism— Partial Keproduction — General Principles. 



Heredity. — This term signifies a highly compounded process. 

 Heredity is not a simple undecomposable force like gravity : 

 a vast assemblage of forces are conveniently grouped together 

 under the word. One day they will, doubtless, be analysed, 

 and referred to ultimate laws, just as Newton analysed the 

 complex movements of the heavenly bodies, showing how they 

 were all the outcome of two simple laws — the law of gravita- 

 tion and the first law of motion. Meanwhile, until this second 

 Newton comes, we shall find it somewhat convenient to regard 

 heredity in the vulgar way — namely, as a separate entity — as 

 a mysterious something which moulds the offspring into the 

 likeness of the parents. Thus I shall, to a large extent, here 

 regard it, setting forth some of the more important facts of 

 the process, without attempting anything further than a super- 

 ficial analysis of them. 



Man multiplies sexually, but there are other methods of 

 reproduction, and it would be impossible to obtain any philo- 

 sophical insight into the principle of heredity without duly con- 

 sidering them. All the various methods fall under two main 

 heads — asexual and sexual. 



Asexual. — The most simple variety of asexual reproduction 

 is by simple division, such as so commonly occurs among 

 unicellular organisms, animal and vegetable. Complex organ- 

 isms, however, may be thus reproduced, as, for instance, the 

 fresh-water worm (nais) ; for, if it be cut into many pieces, 

 each will grow into the likeness of the mature organism. 

 This method by division, and that by " budding," are essen- 

 tially the same. The bud of a plant can, as we know, repro- 



