672 LIFE : OUTLINES OF GENERAL BIOLOGY 



might be a primitive brood-chamber or gonocoel, as it is indeed in 

 the type mentioned. A food-canal could be dispensed with if the 

 peripheral cells were holophytic or if they were able to capturo 

 food-particles by means of fiagella. 



But whatever the precise order of their appearance may be, there 

 is no doubt that in the racial evolution of animals, as now in their 

 individual development, organs appear gradually. Before there 

 Mere defmite nervous cells there was irritability, as we see in the 

 muscle-cells of some sponges; before there was nervous tissue, there 

 were scattered nerve-cells, as in Hydra; before there were nervous 

 organs there was a nerve-network or nerve-plexus, as in sea- 

 anemones. In other words, functions arose and still arise before 

 organs. The attainment of organs implies specialisation of parts, a 

 concentration of functions in particular areas of the body, and a 

 certain degree of circumscribed independence, though it is an even 

 larger fact that the organs of the body work into one another's 

 hands. Among plants there are fewer definite organs than among 

 animals; the lower plants or Thallophytes hardly rise above the 

 level of histonal or tissue organisms; and this condition lingers even 

 among liigher forms, though these have also attained to definite 

 organs, such as leaves, roots, shoots, and the parts of the flower. 

 A tendril, a fly-trap, the pitcher of a Nepenthes, a nectary, may be 

 taken in illustration of plant organs. Morphologically regarded, 

 however, plants do not rise beyond the structural level of Coelentera 

 in the Animal Kingdom. 



Differentiation and Integration. — Many a sponge has an 

 intricate structure, but one piece of its body is like any other piece. 

 Compared with an animal of higher degree, such as a fish, it shows 

 very little local division of labour or differentiation. In the fish there 

 are many specialised parts, while the sponge is very homogeneous 

 in spite of its intricacy. In many cases a portion of the sponge's 

 body can be cut off and bedded out, with the result that it grows 

 an entire sponge. This is not possible with a more differentiated 

 animal. If the comparison be permitted, a railway engine of 1930 

 shows great differentiation compared with Stephenson's first loco- 

 motive of 18 14. Differentiation is the structural side of division of 

 labour. 



But when the contrast between sponge and fish is pressed farther, 

 it is plain that the fish is much more of a unity than a sponge. Its 

 parts are more closely knit together and more adequately subordi- 

 nated to the life of the whole. This kind of progress is called integra- 

 tion; it means a unifying, co-ordinating, and harmonising of the 

 parts of the body. The modern locomotive is much more under 

 control than the locomotive of a century ago ; it is more integrated. 

 In the physiological chapter the question rises how the integration 



