266 THE HUMAN SPECIES 



will-power which has been man's wonder and admiration from 

 the earliest times. 



From the description of the comparative anatomy and 

 physiology of the nervous system, it has already been made 

 clear that the highest development of the animal mind is reached 

 in the Vertebrates, which, in addition to sense-organs and peri- 

 pheral nerve-fibres, have a central nervous system (brain and 

 spinal cord) completely enclosed in a bony covering. It is this 

 great central nervous system, with its wealth of ganglion cells, 

 that forms the basis of the unquestioned superiority of the 

 vertebrate mind over the invertebrate. This applies in a relative, 

 not an absolute sense to the contrast between the highest 

 mammals and man in precisely the same manner as has been 

 seen in the details of comparative psychology. 



Special Comparative Psychology. 



The fact admits of no dispute that sensibility, which we 

 have traced from the simple irritability of protoplasm through 

 the sense-organs right up to the centralisation of the nervous 

 system and the conscious perception of the higher Vertebrates 

 and man, is the original source of all knowledge ; but it is 

 equally certain that a higher phase of the mind-life can only be 

 reached when the impressions derived from the external world 

 are worked up in the nerve-centres. 



Ideation, namely, the forming of a mental picture of an 

 external object, is the first step in advance which the mind has 

 to make so as to orientate itself to the outer world. But the 

 organism only makes this advance step by step. When one 

 considers the perfectly designed cell-structure of the Radiolaria 

 which has adhered for thousands of years to the same funda- 

 mental plan, one is compelled to agree with Haeckel that these 

 unicellular Protista are capable of unconscious ideation. 



Unconscious ideation is found too in the common mind-life 

 of the " cell-communities " (sponges, polyps) as well as in those 

 numerous Invertebrates which have no enclosed central nerve- 

 organ but only ganglia connected on either hand with sensory 

 and motor nerve-fibres. Conscious ideation first occurs in 

 animals with central nerve-organs, not merely in the lower and 

 higher Vertebrates but also in some of the better developed 



