300 EVOLUTION, GENETICS, AND EUGENICS 



ment and Heredity, distinguishes two kinds of reproductive processes: 

 ^^somatogenic and cytogenic, the former including asexual multiplication 

 by fission or budding in which the body itself divides to produce off- 

 spring that are essentially multicellular fragments of itself. Cyto- 

 genic reproduction (cytogony) on the other hand, is effected by means 

 of unicellular germ-cells which by growth and division may build up 

 a new multicellular body." 



The essential feature of all reproductive processes is that some por- 

 tion of the specific living substance of a parent is isolated from the 

 parent, and that the isolated piece, whether a single cell or a mass of 

 cells, carries in itself representatives of all, or at least of many, of the 

 hereditary peculiarities of the stock from which it is derived. In 

 somatogenic reproduction we have a comparatively simple process of 

 reconstitution of a whole organism from a part of an organism. When 

 a Hydra buds off an offspring, we can understand why it should re- 

 semble the parent, for it is obviously no more than a part of the parent's 

 body which becomes separated from the parent and grows independent- 

 ly. It is really a continuation of the parent's body. This kind of 

 reproduction is so uncompUcated that it can be simulated artificially. 

 All we need to do is to cut a Hydra into a number of pieces, say a dozen 

 or more, and each piece will grow into a complete Hydra. Somatogenic 

 reproduction has been extensively practiced in agriculture and horti- 

 culture. If an individual tuber, vine, or tree is found to have a par- 

 ticularly fine body, this body is merely subdivided and each part 

 planted in the ground or grafted upon another individual so as to 

 perpetuate that body indefinitely. Thus our very best potatoes are 

 aU the product of subdividing some one very fine original tuber; all 

 Concord grapes are said to be the result of grafting portions of the 

 body of an original vine produced in Concord, Massachusetts; and 

 all navel oranges are said to be a continuation by grafting of the body 

 of a single tree. There is very Uttle variability among individuals 

 produced in this way, and what Uttle variabiHty there is seems to be 

 entirely due, except in rare instances of bud mutation, to differences 

 in the environment that are not passed on to successive generations. 

 The feature that makes this type of reproduction so useful in agri- 

 culture, namely, that the race remains constant in its somatic expres- 

 sion, makes it a very inefficient evolutionary mechanism, for the very 

 essence of evolution is change or progress. From what we have said 

 about somatogenic reproduction it will have been noted that it involves 

 no sexual processes; in fact, it has commonly been called asexual 

 reproduction. 



