THE GROWTH OF BIOLOGICAL IDEAS 159 



afterward the process was found to be essentially the same in 

 animals. Each chromosome divides longitudinally at cell 

 division, and of the two halves one goes into each daughter 

 cell to help reconstitute a new nucleus. About the same 

 time the German biologist Oscar Hertwig made the momen- 

 tous discovery that the essential feature of fertilization is 

 the fusion of two nuclei, one derived from each parent. It 

 was in the eighties that the Belgian zoologist Edouard van 

 Beneden made one of the most fundamental discoveries of 

 cell science: each nucleus in the body contains two packs of 

 a definite number of chromosomes, the number beino;- con- 

 stant throughout all the cells of the body in each species, ex- 

 cept the spermatozoon and egg, which have only one pack 

 each. The significance of fertilization now began to become 

 apparent; it brought two packs together again. 



People were not slow to see that the extraordinarily precise 

 behavior of the chromosomes must indicate some function of 

 significance for life; and it was suggested that they were con- 

 nected with heredity. So they are, and the knowledge that 

 would have proved it was already lying on the dusty shelves 

 of the libraries of Europe. But no one read the necessary 

 paper. An almost unknown Austrian biologist, the monk 

 Gregor Mendel, had written it in 1866. It had been pub- 

 lished in an obscure journal and sent to London and else- 

 where; but scarcely anyone paid any attention. His paper 

 was independently rediscovered in 1900 by three scientists in 

 different parts of Europe; and it was at once realized that a 

 very important discovery had been made, so important, in- 

 deed, that the study of heredity is to this day often called 

 Mendelism. 



Mendel worked mainly with edible peas, which he grew 

 in the garden of his monastery. His experiments were novel 

 in that he crossed plants differing in one or a few sharply 

 contrasting characters; and these he followed through, gen- 

 eration by generation, always counting accurately the number 

 of plants showing each character. It was particularly his 

 analysis of the ratios in which the characters reappear that 



