Cell Division 41 



development of the higher organisms. In simple cell division, all lines 

 of descent are distinct; any change occurring in a cell which 

 improves its chance of survival, can only affect its own lineal 

 descendants. In sexual reproduction, every individual has many 

 ancestors. It may seem a very small change to have two parents 

 instead of one; but the effect over a number of generations is over- 

 whelming. If one individual receives contributions to his genes from 

 two parents, four grandparents, and eight great grandparents, it is 

 easy to see that the number of ancestors, say twenty or thirty genera- 

 tions back, which may be represented in one or more of the genes, 

 may be enormous. 



Ancestral trees which show the 'descendants' of an individual are 

 extremely misleading. They leave out a great many of the collateral 

 ancestors of succeeding generations. They would be much more 

 informative if written backwards, so that a man or woman could 

 see who are his four grandparents, his eight great-grandparents, and 

 so on, up to 1,048,576 ancestors of the twentieth generation. Even 

 allowing for a considerable amount of duplication, as for example 

 the likelihood that not all these lines of descent are distinct, it is 

 obvious that most of us are descended from practically the whole 

 population of our country twenty generations back, or a mere five or 

 six hundred years ago. 



It is not difficult to see that sexual reproduction is in the long run 

 a very effective device for spreading and uniting all the useful and 

 desirable characteristics in a population. The fact that all the higher 

 forms of life use sexual reproduction shows that it has been an 

 important factor in their development. 



In the chromosomal filaments we come near to the core of the 

 processes of life. Strung along them are the genes which we must 

 regard as molecular patterns which are capable of 



(1) maintaining their identity through many generations of living 

 things, 



(2) of being duplicated in the processes of cell division, 



(3) of controlling the processes whereby a new individual is formed 

 from a fertilized cell. 



We may now ask what is the nature of the substance which has 

 these properties. It was discovered by Miescher in 1871 that the 

 nuclei of cells contain a substance which he called nucleic acid. It 

 was not a protein, but a phosphate of a particular and, at the time, a 

 rather unusual, kind of sugar, called deoxyribose. The whole com- 

 pound is called deoxyribonucleic acid, abbreviated to dna. The 

 sugar and phosphate groups occur in equal numbers and it is now 

 known that they are arranged in long chains, consisting of alternate 



