THE LOWER VERTEBRATES 



159 



Now it seems useful to try to make a curve showing in a graphic 

 way with what types certain features made their appearance 

 in the animal kingdom ; showing the gradual increase of bodily 

 complexity, especially as regards brains; showing the gradual 

 increase of intelligence, of ful- 

 ness and freedom of life, of 

 parental care, of emotional 

 activity,, and so on. The 

 teacher should elaborate this 

 curve for himself, filling it with 

 meaning, and he should, more 

 or less, keep it to himself except 

 in so far as the general ideas it 

 expresses saturate through all 

 his teaching. We may confine 

 our illustration of it to Back- 

 boned Animals. 



The uncompleted perpen- 

 diculars may indicate extinct 

 types, lost races. The dotted 

 line W indicates the level of the 

 water, Amphibians marking 

 the transition to ten a fir ma. 

 From the higher levels, how- 

 ever, there are returns to the 

 water, e.g. sea-snakes, pen- 

 guins, whales. In a general 

 way it may be said that the 

 complexity and unification of 

 the body is indicated by the 

 successive perpendiculars, as 

 well as degree of intelligence, 

 parental care, and so on; but 

 it must be remembered that 

 in each class there are always 

 certain paths. 



1. Indicates the level at which skulls are first seen. 



2. Indicates the level at which jaws are first seen. 



3. Indicates the level at which digits are first seen. 



4. Indicates the level at which a four-chambered heart 



begins. 



5. Indicates the level at which " warm-bloodedness " 



begins. 



6. Indicates the level at which convolutions on the 



brain are first seen. 



types which go very far along 



