r ;8 EVOLUTION, GENETICS, AND EUGENICS 



lission processes in which the entire parent body is distributed among 

 the progeny. Gemmidation is a peculiar form of somatogenic repro- 

 duction found among sponges, involving the gathering together of 

 samples of all the various kinds of cells into compact little balls called 

 gemmitles, which survive the dead parent and escape to form new 

 sponges when the parent body disintegrates. 



Artificial propagation of both plants and animals is made possible 

 through the ability of many organisms to regenerate a whole new in- 

 dividual from a small part of a parent organism. Thus sponges may 

 be cut up into minute fragments and distributed like seeds upon a slab 

 of concrete, each fragment growing up into a sponge of the parental 

 type. Everyone is familiar with the fact that gardeners propagate 

 many plants by planting cuttings, and that potato tubers (enlarged 

 parts of the underground stem) may be cut into as many fragments as 

 there are "eyes" and that each will produce a plant with tubers like 

 the parental tuber. 



All the cells involved in somatogenic reproduction are the product 

 of ordinary mitotic division, which maintains so painstakingly the 

 hereditary organization of the species. No wonder, then, that this 

 mode of reproduction makes for a high degree of constancy of type. 

 Apart from the effects of differences in the environment, which appear 

 not to be inherited, there is nothing about somatogenic reproduction 

 to favor diversity or change. If one desires to study pure heredity 

 uncomplicated by sex and its diversifying effects, one should study 

 successive generations produced by somatogenic reproduction. 



Varieties of cytogenic reproduction. — All those forms of reproduc- 

 tion in which single cells become separated from parent bodies to give 

 rise to new individuals fall into this category. The first and simplest 

 form of cytogenic reproduction is spore formation. Among plants, 

 reproduction by spores is well-nigh universal, though many also make 

 use of gametes. A spore is nothing more than a small cell produced by 

 mitosis from previous cells. Spores are commonly motile, some being 

 furnished with flagella by means of which they travel through the 

 water, others being so light as to be wind-borne. To reproduce a new 

 organism, a spore, after a period of rest, merely divides and redivides 

 and thus forms a new multicellular body. 



In some of the lower plants motile spores, all visibly similar, swim 

 about actively in swarms, and then pair off two by two and fuse to 

 form zygotes from which new plants develop. These mating cells are 

 called gametes. This is the most primitive expression of sex in plants. 



