148 THE PATH OF SCIENCE 



know it, he thus originated that branch of knowledge in 

 which the theory of evolution would one day find its firmest 

 basis. De Buffon, an imposing figure of eighteenth century 

 science, considered that a certain amount of change occurred 

 in the form of organisms with the passage of time, but he did 

 not formulate any systematic theory or explain the causes. 

 Near the end of the century Immanuel Kant, the great phi- 

 losopher, allowed the possibility of evolution in his Critique 

 of Judgmentj and Charles Darwin's grandfather was already 

 a firm believer in the gradual adaptation of organisms to their 

 needs through the inheritance of what were later to be called 

 acquired characters. So also was the brilliant though specu- 

 lative Lamarck, although his ideas on the subject did not 

 attract a lot of attention at the time. More important than 

 any of these for the firm foundation of the theory of evolution 

 was a clergyman and economist named Thomas Malthus. He 

 was not himself a student of evolution or even of biology; he 

 was interested in the pressure of human population on the 

 available means of subsistence. But his writings on the sub- 

 ject were later to influence both Charles Darwin and Alfred 

 Russel Wallace, whose theory of evolution was to have such a 

 profound effect on biological thought sixty years later. 



Modern ideas on evolution are closely bound up with our 

 knowledge of heredity, but in the eighteenth century that 

 subject was illuminated by only a single glimmer of light. 

 Just the very beginnings of knowledge were visible in Joseph 

 Koelreuter's experiments on hybridization. But no one then 

 could aruess what wonders Mendel and his successors would 

 do with the numerical analysis of results in this field. Koel- 

 reuter made a start along a line that did not begin to in- 

 fluence thought on the causes of adaptation until long after 

 the main battle for evolution had been fought and won. 



Understanding of the processes of reproduction came very 

 slowly. A Dutch student, Hamm, discovered spermatozoa 

 in 1679. In the next century Spallanzani filtered semen and 

 showed that fertilization cannot take place unless spermatozoa 

 are present in it; but he did not conclude that they were the 



