340 PROGRESS OF SCIENCE IN THE CENTURY. 



The bird is more of a unity than a sponge ; its parts 

 are more closely knit together and more adequately 

 subordinated to the life of the whole. We call this 

 kind of progress integration. Differentiation in- 

 volves the acquisition of new parts and powers, these 

 are consolidated and harmonised as the animal be- 

 comes more integrated.* 



Stephenson's " Puffing Billy " was a lower organ- 

 ism than a locomotive of 1901 ; it showed less com- 

 plexity of usefully functional parts, and it was less 

 under unified control 



Our point is that we are continually using words 

 like " organism," " development," " differentiation," 

 " integration," "individuality," "character," " adap- 

 tation," and so on, using them lightly as if there 

 were no difficulties hidden in them and that there- 

 fore such general philosophic works as the two we 

 have named are of great value in expressing at least 

 an attempt to criticise and clarify the categories 

 which even the purest of " pure anatomists " must 

 use in spite of himself. Neither Spencer nor 

 Haeckel would regard his masterpiece of 1866 as 

 final ; indeed Spencer in his last years began to 

 re-edit The Principles of Biology; and it is plain 

 that the criticism of categories must develop as the 

 science does, but the fact remains that there are few 

 biological books of more recent date which come near 

 those of Spencer and Ilaeckel in extent or lucidity 

 of outlook. 



Change of Function. Division of labour involves 

 restriction of functions in the several parts of an 

 animal, and no higher animals could have arisen if 

 all the cells had remained with the many-sided 

 qualities of Amoebae. Yet we must avoid thinking 

 8ef the writer's Outline* of Zoology, 3rd edition, 1899, 



