216 



EVOLUTION AND ANIMAL LIFE 



A 



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

 the queen bee lays both fertilized and unfertilized eggs. From 

 the fertilized eggs hatch the workers, which are rudimentary 

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

 from the unfertilized eggs hatch only males the drones. 

 Many generations of plant lice are produced each year parthe- 

 nogenetically that is, by unfertilized females. This subject 



will be discussed at 

 greater length later 

 B (BH1 ^-^ in this chapter. 



The modes of 

 generation, or re- 

 production, or mul- 

 tiplication, as this 

 making the begin- 

 nings of new indi- 

 viduals may be 

 variously called, so 

 far referred to, may 

 be grouped into a 

 category called asex- 

 u a 1 generatio n. 

 In an examination 

 of the lives of the 

 simplest and but 

 slightly complex 

 kinds of animals we 

 find that even 

 among almost the 

 very simplest of or- 



FIG. 123. Gregarinida*. A, A gregarinid, Actinocephalus 

 oligacanthus, from the intestines of an insect (after 

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

 ritlium oriforme, from liver of a guinea-pig (after Leut- 

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

 spore-forming by Gregarina polytnorpha. (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. 



Paramcecium, 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 



