270 THE INDUCTIONS OF BIOLOGY. 



Instead of there being, as in the cases most familiar to us, a 

 constant recurrence of the same form, there is a cyclical 

 recurrence of the same form. These two distinct processes of 

 multiplication, may be aptly termed homogenesis and hetero- 

 genesis.* Under these heads let us consider them. 



There are two kinds of homogenesis, the simplest of them, 

 probably once universal but now exceptional, being that in 

 which there is no other form of multiplication than one result- 

 ing from perpetual spontaneous fission. The rise of distinct 

 sexes was doubtless a step in evolution, and before it took 

 place the formation of new individuals could have arisen 

 only by division of the old, either into two or into many. 

 At present this process survives, so far as appears, among 

 Bacteria, certain Algce, and sundry Protozoa; though it is 

 possible that a rarely-occurring conjugation has in these cases 

 not yet been observed. It is a probable conclusion, how- 

 ever, that in the Bacteria at any rate, the once universal 

 mode of multiplication still survives as an exceptional 

 mode. But now passing over these cases, we have to 



note that the kind of genesis (once supposed to be the sole 

 kind), in which the successive generations are alike, is 

 sexual genesis, or, as it has been otherwise called gamo- 

 genesis. In every species which multiplies by this kind of 

 homogenesis, each generation consists of males and females; 

 and from the fertilized germs they produce the next generation 

 of similar males and females arises : the only needful qualifi- 

 cation of this statement being that in many Protophyta and 

 Protozoa the conjugating cells or protoplasts are not distin- 

 guishable in character. This mode of propagation has the 

 further trait, that each fertilized germ usually gives rise to 

 but one individual the product of development is organized 

 round one axis and not round several axes. Homogenesis in 



* Unfortunately the word heterogenesis has been already used as a synonym 

 for " spontaneous generation." Save by those few who believe in " spontane- 

 ous generation," however, little objection will be felt to using the word in a 

 sense that seems much more appropriate. The meaning above given to it 

 covers both Metagenesis and Parthenogenesis. 



