I A CHAPTEE IN DARWINISM 15 



the proportions of the animals or plants on which he 

 operates, so in inconceivably long periods of time 

 has this struggling of varieties, and the consequent 

 natural selection of the fittest, led to the production, 

 from shapeless primitive living matter, of all the end- 

 less varieties of complicated plants and animals which 

 now people the world. Countless varieties have died 

 out, leaving only their modified descendants to puzzle 

 the ingenuity of the biologist. 



Of the tens and hundreds of thousands. of interme- 

 diate forms we know nothing by direct observation. 

 They have perished as better-fitted forms ousted them 

 in the never-ending conflict. But we feel sure that 

 they once were in existence, and can infer what was 

 their structure, and what were their peculiarities, by 

 the study of the structure and attributes of their now 

 living descendants. 



If all the forms of life at present living are the 

 modified offspring of a smaller number of ancestral 

 forms which have died out, and if these again were 

 the modified descendants produced by ordinary parent- 

 age of a single original living thing — then the whole 

 series of forms that have ever lived could, if we had 

 them before us, be arranged in the form of a great 

 family-tree — the various branches presenting a perfect 

 gradation of forms arranged one after another, lead- 

 ing down from the terminal twigs (which would repre- 

 sent the latest forms produced) to larger and larger 

 branches, until the common trunk representing the 

 original ancestor would be reached. Our actual means 

 of observing the genealogical affinities of different 



