162 THE PATH OF SCIENCE 



dence suggested very strongly that some of the genes, cor- 

 responding to a short length of one chromosome, were the 

 "wrong" way around. It occurred to T. S. Painter and his 

 associates at the University of Texas to look at the giant 

 chromosomes of these particular specimens. In his micro- 

 scope he saw for the first time concrete proof of the chromo- 

 some theory of heredity: the thick and thin marks were in 

 fact arranged the wrong way around in part of the chromo- 

 some concerned. 



Our modern understanding of heredity has thrown a strong 

 light on the causes of evolution without, as yet, providing an 

 explanation that commands general assent. Back in 1858 a 

 theory of causes had been put forward by Charles Darwin 

 and Alfred Russel Wallace. The idea had occurred to them 

 independently. Both had read Malthus on population (page 

 148). In Darwin's mind the idea formed gradually over a 

 long period of years; into Wallace's it flashed suddenly while 

 he was suffering from an attack of malaria in the East Indies. 

 They saw that organisms produce far more offspring than 

 can survive; that those offspring differ among themselves; and 

 that, on the average, those that chance to be the best adapted 

 to their environment will survive. These fittest individuals 

 would pass on their characters to their offspring, and thus 

 the race would gradually evolve. The publication of The 

 Origin of Species in 1859 is a landmark in the history of 

 biology. 



Nowadays we can see that Darwin's chief service to science 

 was the production of a mass of evidence that evolution has 

 occurred. That mass of evidence has been multiplying ever 

 since, and the fact of evolution is not today in doubt. But 

 although he studied variation and wrote a large book on it, 

 Darwin never found out how variations are inherited. It 

 was Mendel who did that. It is interesting to speculate on 

 what would have happened if Mendel had sent a copy of his 

 paper to Darwin. The latter, however, died without ever 

 hearing of Mendel's work, and real study of the causes of 



