THE STUDY OF STRUCTURE. 339 



logic are classics of which the nineteenth century 

 might have been prouder than it was. They are 

 monumental attempts to systematise and clarify the 

 general conceptions which underlie all biological 

 thinking and research. 



Let us take a simple illustration. We say that 

 one animal is " higher " than another, what do we 

 mean ? Merely, that it is liker ourselves ? Or is there 

 more precision in our standard ? The answer is to be 

 found in the words " differentiation " and " integra- 

 tion " ; the higher animal is more differentiated and 

 more integrated than the lower. And what the two 

 big words mean is made plain in the classics referred 

 to. 



The progress of the individual, and of the race, 

 is from simplicity to complexity. When we think 

 over the animal series we also notice that before defi- 

 nite nervous organs appear there is diffuse irritabil- 

 ity, before definite muscular organs appear there is 

 diffuse contractility, and so on. In other words, 

 functions come before organs. The attainment of 

 organs implies specialisation of parts, or concentra- 

 tion of functions in particular areas of the body. 



Contrast a frog with Hydra, and one of the great 

 facts about the evolution of organs is illustrated. 

 Among the living units which make up a frog, there 

 is much more division of labour than there is among 

 those of Hydra. An excised representative sample 

 of Hydra will reproduce the whole, but you cannot 

 perform this experiment with the frog. Now, the 

 structural result of this physiological division of 

 labour is differentiation. The animal, or part of 

 it, becomes more complex, more heterogeneous. 



Contrast a bird and a sponge, and another great 

 fact about the evolution of organs is illustrated. 



