2io PIONEERS OF EVOLUTION PART 



the mammals immediately beneath him is proven, 

 the connection of the mammals with the lowest 

 invertebrate may be assumed as also established. 

 Speaking only of vertebrates, the brain being, whether 

 in fish or man, the organ of mental phenomena, 

 how far does its structure support or destroy the 

 theory of mental continuity? In Man's Place in 

 Nature, and its invaluable supplement, the second 

 part of the monograph on Hume, this subject is 

 expounded by Huxley with his usual clearness. In 

 the older book he traces the gradual modification 

 of brain in the series of backboned animals. He 

 points out that the brain of a fish is very small 

 compared with the spinal cord into which it is 

 continued, that in reptiles the mass of brain, rela- 

 tively to the spinal cord, is larger, and still larger in 

 birds, until among the lowest mammals, as the 

 opossums and kangaroos, the brain is so increased 

 in proportion as to be extremely different from that 

 of fish, bird, or reptile. Between these marsupials 

 and the highest or placental mammals, there occurs 

 ' the greatest leap anywhere made by Nature in her 

 brain work.' Then follows this important statement 

 in favour of continuity. 



^ ' As if to demonstrate, by a striking example, the 

 impossibility of erecting any cerebral barrier between 

 man and the apes, Nature has provided us, in the 

 latter animals, with an almost complete series of 

 gradations from brains little higher than that of 

 a Rodent to brains little lower than that of Man.' 

 After giving technical descriptions in proof of this, 

 and laying special stress on the presence of the 

 structure known as the ( hippocampus minor ' in the 



