SCIENCE 



[N. S. Vol. XXXI. No. 784 



two functional systems and differentiation 

 from the other, so that in higher verte- 

 brates the distinction between them may be 

 said to be fundamental both to anatomy 

 and to physiology. 



As children we probably considered the 

 chief distinction between plants and ani- 

 mals to be the ability of the latter to move 

 freely about ; but one of the first lessons in 

 our elementary biology was the correction 

 of this notion by the study of sedentary 

 animals and locomotor plants. Neverthe- 

 less, I fancy that in the broad view the 

 childish idea has the root of the matter in it. 

 The plants and sedentary animals may 

 have their vegetative functions of internal 

 adjustment never so highly specialized and 

 yet remain relatively low in the biological 

 scale because their relations with the en- 

 vironment are necessarily limited to the 

 small circle within which they first take 

 root, whereas the power of locomotion car- 

 ries with it, at least potentially, the ability 

 to choose between many more environ- 

 mental factors. It is only the free-moving 

 animals that have anji;hing to gain by look- 

 ing ahead in the world, and here only do 

 we find well-developed distance receptors, 

 i. e., sense organs adapted to receive im- 

 pressions from objects remote from contact 

 with the body. And the distance receptors, 

 as we shall see, have dominated and set the 

 direction of the evolution of the nervous 

 system in vertebrates. 



Thus arose the animal head, with its 

 three important functions of feeding, 

 breathing and the recognition of mates and 

 enemies. Parker has recently reviewed- 

 in an illuminating way the earlier stages 

 in the differentiation of the nervous sys- 

 tem and I shall not attempt to go over this 

 ground again, but will take a bilaterally 

 symmetrical segmented animal with a dif- 

 ferentiated head end as the point of de- 



- Parker, G. H., Popular Soience Monthly, 1909. 



parture in an examination of the phylogen- 

 etic history of behavior types. 



On anatomical and zoological grounds 

 zoologists are in the habit of subdividing 

 the animal kingdom in the way roughly 

 suggested by the accompanying scheme. 

 Most of the important groups are naturally 

 arranged in two great phyla which have 

 apparently been quite distinct as far down 

 as the flat worms. One of these, which we 

 may call the articulate phylum, includes 



.CEPhalOCHORDA 



ANNELIDA 



ENTEROPNEUSTA 



TURBCLLARIA 

 COELENTERATA / 



the segmented worms, crustaceans, spiders 

 and insects; the other phylum, after pass- 

 ing through a series of obscure invertebrate 

 stages, largely at the present time extinct, 

 culminates in the true vertebrates. It may 

 be termed briefly the vertebrate phylum, 

 and all of its members, from the lowest to 

 the highest, are sharply distinguished from 

 those of the articulate phylum by several 

 characteristics, among which is the devel- 

 opment of mesodermal gut pouches. All 

 forms above the Enteropneusta have gill 

 clefts, either embryonic or adult, which 

 likewise develop as gut pouches and a dor- 

 sal tubular nervous system, which is de- 

 rived from the mid-dorsal ectoderm and is 

 separated from the gut by a supporting 

 notochord. The articulates, on the other 



