APPENDIX 421 



also are composed of single parts, eacli of wliicli can, when 

 separated from the whole, under certain conditions, grow into 

 a new animal — in sponges this takes place in any fragment 

 separated from the whole and tlirown again into the water. 

 We have thns in all these cases, in the adult animal, not 

 something simple, and this is what I want to emphasize, 

 but an aggregate of similar members. 



Such an aggregation occurs also in animals where it is not 

 usually suspected — for instance, in insects and their alHes, in 

 the group of Arthropoda generally. A fly or a bee is not 

 an originally simple whole, but it is clearly evident I'rom its 

 developmental history that it was evolved from a worm-like 

 creature, i.e. one which consisted of a linear series of similar 

 parts. We recognise this condition still to some extent in the 

 caterpillar, and also in the maggot or simihar larva of other 

 forms. Thus the bee has been evolved from a series of secj- 

 ments originally equivalent. But a unity has been developed 

 in this animal in consequence of the division of labour which 

 has taken place among the segments. The various organs are 

 no longer distributed equally among the several segments of 

 the body, but collected together, united in difterent regions. 

 This applies also to the nervous system, although closer 

 observation shows that here there is not yet the unity whicli 

 might be expected. For the several nerve-centres which are 

 present in the body of an insect have more or less different 

 functions : stimulation of one group of nerve-centres alfects 

 the sexual organs, of another the locomotive organs, and so on. 

 I only say therefore that in the body of an insect we have 

 a composite structure which has arisen from a number of 

 equivalent segments ; in consequence of division of labour 

 these segments have been united into a single body, differen- 

 tiated into several groups composed of dilferent organs and 

 lying one behind another. 



The same is the case in our own bodies. P2ven tlie 



