7 1 GENETICS AND EVOLUTION 



bage cross made by Karpechenko. Although radishes and cabbages be- 

 long to different genera, each has 18 chromosomes. The resulting hybrid 

 also had 18 chromosomes, 9 from the radish parent and 9 from the cab- 

 bage parent. The radish and cabbage chromosomes were not sufficiently 

 alike to permit synapsis to occur normally and the hybrid was almost 

 completely sterile. The chance distribution of the chromosomes led to 

 the formation of a few eggs and sperm that had 18 chromosomes each, 

 and the union of such eggs and sperm resulted in a plant with 36 

 chromosomes. This new plant was fertile; in meiosis the homologous 

 radish chromosomes paired with each other and the homologous cab- 

 bage chromosomes paired with each other. The new hybrid had some 

 of the characteristics of each of its parents and bred true for them. It 

 was not valuable commercially, however, for it had roots like a cabbage 

 and a top like a radish. Since this hybrid could not be crossed readily 

 with either of its parental species, Raphanus sativus, the radish, or 

 Brassica oleracea, the cabbage, Karpechenko named this new, experi- 

 mentally produced genus Raphanobrassica. 



There are many other examples of species of plants produced by 

 hybridization and chromosome doubling, but this process appears to 

 have played a negligible role in the evolution of animals. Two explana- 

 tions of this have been advanced: the gametes of animals are more 

 sensitive to imbalances of chromosomes and are nonviable unless a nor- 

 mal haploid set is present; since the sexes are separate in most animals, 

 the random segregation of several pairs of sex chromosomes in a poly- 

 ploid animal might lead to the formation of sterile combinations. 



305. The Origin of Life 



The modern theories of mutation, natural selection and popula- 

 tion dynamics provide us with a satisfactory explanation of how the 

 present-day animals and plants evolved from previous forms by descent 

 with modification. The question of the ultimate origin of life on this 

 planet has been given serious consideration by many different biologists. 

 Some have postulated that some kind of spores or germs may have been 

 carried through space from another planet to this one. This is unsatis- 

 factory, not only because it begs the question of the ultimate source of 

 these spores, but because it is extremely unlikely that any sort of living 

 thing could survive the extreme cold and intense irradiation of inter- 

 planetary travel. 



The concept that the first living things did evolve from nonliving 

 things has been put forward by Pfliiger, J. B. S. Haldane, R. Beutner, 

 and especially by the Russian biochemist, A. I. Oparin, in his book, 

 The Origin of Life (1938). The earth originated some 2.5 billion to 4.5 

 billion years ago, either as a part broken off from the sun or by the 

 gradual condensation of interstellar dust. Most authorities seem agreed 

 that the earth at first was very hot and molten, and that conditions 

 consistent with life arose only one billion or perhaps a billion and a 

 half years ago. At that time the earth's atmosphere contained essentially 

 no free oxygen; all the oxygen was combined as water and as oxides. 



