234 



EVOLUTION AND GENETICS 



carrion. Seed eaters have thick, powerful beaks, but in in- 

 sectivorous species the beak is more slender (Fig. 142). 



Protective Adaptation. Reproduction. A simple protection 

 for species which are preyed upon by others is the production of 

 enough offspring to maintain the species in spite of its constant 

 loss. Among many species this adaptation of reproduction takes 

 the place of more active defenses. The oyster, for example, lays 

 approximately 16,000,000 eggs each year, and many fishes are said 

 to lay millions. In other animals this enormous rate of repro- 

 duction is supplanted by the production of less young, with paren- 



Ir-e 



mx 



Fig. 141. — The head and mouthparts of a mosquito, Anopheles sp. a, antennae; 

 Ir-e, labrum-epipharynx; h, hypopharynx ; m, mandibles; mx, maxillae; I, 

 labium; mp, maxillary palpi. (After Nuttall and Shipley, from Comstock's 

 Introduction to Entomology, with the permission of the Comstock Publish- 

 ing Company.) 



tal care to aid them in reaching maturity. Such adaptations 

 often involve structural modifications of the organism. They may 

 result in viviparity, i.e., the production of living young, so that 

 the inert egg and tender embryo are not subjected to the vicissi- 

 tudes of independent existence. In extreme degrees viviparous 

 species bear young when sufficiently developed to care for them- 

 selves to a marked degree, while in others Httle more than the 

 embryonic period is eliminated from the independent existence of 

 the individual. Mammals are the best known viviparous ani- 

 mals, and in this group parental care is also highly developed. 

 Normally oviparous groups, such as the fishes and insects, may 

 also include viviparous species. Viviparity is not essential to 



