HOW ORGANISMS REPRODUCE THEMSELVES oqj 



Cytogenic reproduction, on the other hand, may be either asexual 

 or sexual. The sexual mode of reproduction is characterized by the 

 fact that two germ cells or gametes unite to form a combination cell 

 or zygote. A curious aberration of the gametic mode of reproduction 

 is found rather widespread throughout both animal and plant groups, 

 known as parthenogenesis, in which the gametes of the female {ova) may 

 start developing without being activated by the male gametes {sper- 

 matozoa) and produce normal offspring. As is to be shown in a subse- 

 quent chapter, parthenogenetic offspring in any given species are nearly 

 always all of one sex, sometimes being females, sometimes males. The 

 usual type of cytogenic reproduction, however, is what is commonly 

 called sexual or gametic reproduction. 



SEXUAL OR GAMETIC REPRODUCTION 



The prevailing type of reproduction from the highest to the lowest 

 organisms is the sexual type, and it is now believed that in this process 

 we have the machine not only for multiplying individuals, but for pro- 

 ducing variability and for passing on new characters to future genera- 

 tions. In short, sexual reproduction appears to be, in last analysis, the 

 evolutionary machine. Therefore, if we are ever to understand the 

 mechanics of evolution, we must expect to get our knowledge from ob- 

 servation and analysis of the processes involved in the germ-cell cycle. 

 The germ cells behave very differently from body cells in several ways, 

 especially in the processes known as maturation and fertilization. 

 These peculiar processes will be described in detail in the next chapter. 



Sex as an aid to evolution. — In addition to the fact that males and 

 females are specialized individuals, each performing a different repro- 

 ductive function and performing such function more effectively than 

 it could be done were there only one sex, there is now no question but 

 that sexual reproduction is vastly superior to asexual with respect to 

 its capacity to produce far greater diversity in progeny. With the 

 far greater range of diversity there is a much greater chance for the 

 organism to produce bodily conditions that will be adapted to changes 

 in the environment and thus continue to live and evolve. Essentially 

 the union of gametes means the bringing together of two different 

 strains of protoplasm, each of which has some peculiarities that the 

 other lacks. Such a combination may result in associating in one 

 individual two relatively poor or unfortunate traits, each of which alone 

 might be no very serious detriment, but which together make for a very 

 serious condition. Conversely, there may readily result a fortunate 

 combination of two excellent characters that together might make a 



