216 



EVOLUTION AXD ANIMAL LIFE 



B 



young from eggs which have not been fertihzed. For example, 

 the queen bee lays both fertilized and unfertilized eggs. From 

 the fertilized eggs hatch the workers, which are rudim.entary 

 females, and other ciueens, which are fully developed females; 

 from the unfertilized eggs hatch only males — the drones. 

 Many generations of plant lice are produced each j^ear parthe- 

 nogenetically — that is, by unfertilized females. This subject 



will be discussed at 

 greater length later 

 in this chapter. 



The modes of 

 generation, or re- 

 production, or mul- 

 tiplication, as this 

 the begin- 

 of new indi- 

 viduals may be 

 variously called, so 

 far referred to, may 

 be grouped into a 

 category called asex- 

 ual generation. 

 In an examination 

 of tlie lives of the 

 simplest and but 

 slightly complex 

 kinds of animals we 

 find that even 

 among almost the 

 very simplest of or- 



making 

 nings 



Fig. 123. — Gregarinida?. A, A gregarimd,^c^mocepAa7?/s 

 oligacanthus, from the intestines of an insect (after 

 Stein); B and C, spore-forming bj^ a gregarinid, Coc- 

 cidium ovifornie, from, liver of a guinea-pig (after Lexie- 

 kart); D, E, and F, successive stages in conjugation of 

 spore-forming by Gregarina polymorpha. (After Kol- 

 liker.) 



ganisms another 

 mode of reproduction obtains, at least occasionally, which 

 demands for its carrying out the mutual action of two distinct 

 individuals. The essential thing in this mutual action is the 

 exchange of nuclear material from one of these individuals to 

 the other; with some of the simplest organisms there is a mutual 

 exchange of nuclear material. 



Paramoecmm., for example, reproduces itself for many gen- 

 erations by fission, but a generation finally appears in which 

 a different method of reproduction is followed. Two individu- 

 als come together and each exchanges with the other a part of 



