432 BIOLOGY AND ITS MAKERS 
As a thinker he is more careless than Huxley, and as a result 
less critical and cxact as a writer. 
There can be no doubt that the germs of evolutionary 
thought existed in Greek philosophy, and that they were 
Fic. 122.—Ernst HaECKEL, Born 1834. 
retained in a state of low vitality among the medizval thinkers 
who reflected upon the problem of creation. It was not, 
however, until the beginning of the nineteenth century that, 
under the nurture of Lamarck, they grew into what we may 
speak of as the modern theory of evolution. After various 
vicissitudes this doctrine was made fertile by Darwin, who 
supplied it with a new principle, that of natural selection. 
The fruits of this long growth are now being gathered. 
After Darwin the problem of biology became not merely 
to describe phenomena, but to explain them. This is the 
