150 EVOLUTIONISTS OF THE EIGHTEENTH CENTURY. 



to the vegetable world, it is not impossible but the 

 great variety of species of animals which now ten- 

 ant the earth, may have had their origin from the 

 mixture of a few natural orders." Elsewhere he 

 speaks of plants as having arisen in the contest for 

 light and air. He carries the idea of sensibility and 

 irritability into plant life, and his theory of plant 

 evolution is similar to that of animal evolution. 



Erasmus Darwin was, however, fully conscious 

 of the limitations of his theory of Evolution ; for in 

 speaking of protective colouring (p. 510), he says: 

 " The final cause of these colours is readily under- 

 stood, as they serve some purpose of the animal, but 

 the efficient cause would seem almost beyond con- 

 jecture." The same question we have seen pro- 

 pounded by Kant at about the same period : " How 

 can purposeful forms of organization arise without 

 a purposeful working cause } How can a work full 

 of design build itself up without a design and with- 

 out a builder } " Of course we do not know whether 

 Darwin had this suggested to him by Kant, but it 

 is exceedingly interesting to see him so clearly state 

 the old, old problem which his grandson later largely 

 solved. 



While this chapter on Generation is a compara- 

 tively small part of the Zooiiomia, we learn that it 

 attracted much attention at the time. Dr. McCosh 

 tells the writer that he read the work while in Edin- 

 burgh. It made a considerable sensation, and was 

 replied to by Thomas Brown, M.D. This reply, 



