432 LAWS OF MULTIPLICATION. 



generation. A modified mode, apparently not fatal to the 

 parents, has been observed in certain of the more developed 

 Infusoria. Our knowledge of these microscopic types is, 

 however, so rudimentary that evidence derived from them 

 must be taken with a qualification. 



Among small animal aggregates of the second order, the 

 first to be considered are of course the Cceleiiterata . A Hydra 

 occasionally devotes a large part of its substance to sexual 

 genesis. In the walls of its body groups of ova, or sperma- 

 tozoa, or both, take their rise ; and develop into masses 

 greatly distorting the creature's form, and leaving it greatly 

 diminished when they escape. Here, however, gamogenesis is 

 obviously supplementary to agamogenesis — the immensely 

 rapid multiplication by budding continues as long as food is 

 abundant and warmth sufficient, and is replaced by gamo- 

 genesis only at the close of the season. A better 

 example of the relation between small siae and active gamo- 

 genesis is supplied by the Planaria, which does not multiply 

 asexually with so much rapidity. The generative system is 

 here enormous. Ova are developed all through the body, 

 occupying everywhere the interspaces of the assimilative 

 system ; so that the animal may be said to consist of a part 

 that absorbs nutriment and a part that transforms that nutri- 

 ment into sperm-cells and germ-cells. Even saying nothing 

 of the probably- early maturity of these animals, and there- 

 fore frequent repetition of sexual multiplication, it is clear 

 that their fertility must be very great. 



The Annulosa, including among them the inferior kindred 

 types, have habits and conditions of life so various that only 

 the broadest contrasts can be instanced in support of the pro- 

 position before us. Of the microscopic forms belonging to 

 this sub-kingdom, the Rotifera may be named as having, 

 along with small bulk, a great rate of sexual increase. Hyda- 

 tina &enta " is capable of a four- fold propagation every twenty- 

 four or thirty-hours, bringing forth in this time four ova, 

 which grow from the embrj'o to maturity, and exclude tlieir 



