EVOLUTION AND GENERATION 



259 



living things is the simple result of the separation of portions of the 

 mass, which are at once new beings possessing all the powers and pro- 

 perties of their parent. Among the Infusoria, the highest division of 

 the Protozoa, there are endless varieties of form, most of them moving 

 freely in the fluid in which they live, by the aid of fine hair-like pro- 

 jections (cilia), although some of them have a stationary life, being 

 attached to stones or other bodies. Their reproductive powers are always 

 active, and result in the growth of buds, which project from their bodies, 

 become severed from their parents, 

 and enjoy an independent exist- 

 ence, giving origin to new lives 

 by the simple process of budding 

 in their turn. Division of the 

 organisms is another method of 

 multiplication which is common. 

 The paramecium, for instance, has 

 been seen to divide into several 

 parts, which go on dividing every 

 twenty-four hours. Monads, which 

 are the smallest of infusorial ani- 

 malcules, exhibit phenomena closely 

 allied to those which have been 

 described in the early changes in 

 the mammalian ovum, i.e. multi- 

 plication by cleavage. A small 



fisjnrp is nWrvprl in flip ppll Wflll 



at two, sometimes at four, points, 



and by the simple extension of the fissures the creature is converted into 



two or four individuals. 



Alternate Generation. Among the variations, some of which have 

 been described in the function of generation, that of alternate generation 

 is the most remarkable. It has been aptly defined as the production, by 

 an animal, of an offspring which at no time resembles its parent, but 

 which itself brings forth* a progeny which gives rise to other forms still 

 differing from the parent animal, so that the original maternal animal 

 does not meet with its resemblance in its own brood, but in its de- 

 scendants of the second, third, or fourth generation. This paradoxical 

 position of the reproductive function is not exceptional nor even rare. 

 Vertebrate animals are the only class in which it has not been observed. 

 In bell-shaped Polypes, Claviform Polypes, Medusae, Salpse, Vorticellse, 

 and Entozoa it is well known. The last-named class, Entozoa, and 



Fig. 544. Alternate Generation 



1. Chrysaora (Medusa): a, egg; 6, Hydratuba stage; 

 c, Hydra undergoing subdivision ; a", young medusa de- 

 tached. 2. Distomum hepaticum: e, adult; /, egg; g, 

 larva (1st generation); h, redia stage (2nd generation); 

 I, cercaria stage (3rd generation). 



