14 ESSAYS OF A BIOLOGIST 



No ; the remedy for all our difficulties, and indeed 

 the only way in which we can arrive at the possibility 

 of saying whether biological progress exists or no, is 

 to adopt the positive method. 



Let us then begin our survey of biological evolution 

 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 — biologic- 

 ally negligible — in which man has directed his attention 

 to the problem ; and historical record is also absent. 

 The best available evidence is that of palaeontology : 

 here the relative positions of the layers of the earth's 

 crust enables us to deduce their temporal sequence 

 — ^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 organization 

 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 



^ 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. 



