THE NATUEE OF THE SOUL. 105 



biological research in a broad survey, and make 

 strenuous efforts to rind a unifying, natural basis for 

 their knowledge ; in their later years they have found 

 that this is not completely attainable, and so they 

 entirely abandon the idea. In extenuation of these 

 psychological metamorphoses they can, naturally, 

 plead that in their youth they overlooked the difficulties 

 of the great task, and misconceived the true goal ; 

 with the maturer judgment of age and the accumula- 

 tion of experience they were convinced of their errors, 

 and discovered the true path to the source of truth. 

 On the other hand, it is possible to think that great 

 scientists approach their task with less prejudice and 

 more energy in their earlier years — that their vision is 

 clearer and their judgment purer ; the experiences of 

 later years sometimes have the effect, not of enriching, 

 but of disturbing, the mind, and with old age there 

 comes a gradual decay of the brain, just as happens in 

 all other organs. In any case, this change of views is 

 in itself an instructive psychological fact ; because, 

 like many other forms of change of opinion, it shows 

 that the highest psychic functions are subject to 

 profound individual changes in the course of life, like 

 all the other vital processes. 



For the profitable construction of comparative psy- 

 chology it is extremely important not to confine the 

 critical comparison to man and the brute in general, 

 but to put side by side the innumerable gradations of 

 their mental activity. Only thus can we attain a 

 clear knowledge of the long scale of psychic develop- 

 ment which runs unbroken from the lowest, unicellular 

 forms of life up to the mammals, and to man at 

 their head. But even within the limits of our own 

 race such gradations are very noticeable, and the 



