14 ESSAYS OF A BIOLOGIST 



No; the remedy for all our difficulties, and indeed 

 the only way in which we can arrive at the possibil- 

 ity of saying whether biological progress exists or no, 

 is to adopt the positive method. 



Let us then begin our survey of biological evolu- 

 tion in the endeavour to find whether or no progress 

 is visible there. To start with, we must be clear 

 what are the sources of our knowledge on the subject. 



Direct observation of progressive evolution has, of 

 course, not yet been possible in the period — biologi- 

 cally negligible — in which man has directed his atten- 

 tion to the problem; and historical record is also ab- 

 sent. The best available evidence is that of paleon- 

 tology: here the relative positions of the layers of the 

 earth's crust enable us to deduce their temporal se- 

 quence — and naturally, that of the organisms whose 

 fossil remains they embalm — with a great deal of 

 accuracy.^ 



We can scarcely ever observe the direct transition 

 from the forms of life in an older to those in a 

 younger stratum, nor can we absolutely prove their 

 genetic relationship. But in a vast number of cases 

 it is abundantly clear that the later type of organiza- 

 tion is descended from the former — that a group of 

 forms in the younger stratum had its origin in one or 

 more species of the group to which the forms in the 

 older stratum belong. Sometimes, however, as in 



s This holds good, naturally, for any given spot on the earth's 

 crust: once the contained fossils have been carefully examined 

 from a number of series of strata, they enable us to correlate the 

 ages of the members of the different series. 



