Ai.,(iiijii ..hi. luiii i)f Naluicil ll.biury 



SEXUAL DIMORPHISM IN MAMMALS 

 Wapiti deer in Northern Colorado 



poets rather than by scientists, we are made to see at once the resemblance 

 between maleness or femaleness in plants and the corresponding characteris- 

 tics of animals. And this in spite of the great differences between plant be- 

 havior and animal behavior, and in spite of the great differences in the matter 

 oi feeling, which is immeasurably more intense in the highest vertebrates than 

 we can conceive it to be in other species of organisms. The flowering plants 

 deserve at least a chapter for a survey (see Chapter 20). 



In Brief 



Unicellular plants and animals reproduce themselves by cell-division; 

 their protoplasm is potentially immortal. 



Cell-division is an essential feature of development, as well as of growth. 



In multicellular organisms cell-division results in growth, in the healing of 

 injuries, or in the regeneration of lost parts, and in the reproduction of new 

 individuals. 



Many species produce specialized cells, or spores, from which new individ- 

 uals develop; these spores are capable of resisting unfavorable conditions 

 almost indefinitely. 



Many species produce specialized reproductive cells, gametes, which 

 unite in pairs into zygotes; these, in turn, develop into new individuals. 



393 



