288 



THE EVOLUTIONARY CAREER 



Gimthostomata 



Ichthyopsida 



Saiiropsida 



Mammalia 



N(E)C Dn Br J 



. . A or (A) 



Br or (Br) 



Gnathostomata as above 

 an air bladder present 

 Branchiae may change 

 Gnathostomata as above 



N(E)C Dn (Br)J(A) 

 The air bladder is a lung , . (A) 



Gnathostomata as above 



N(E)C Dn (Br)J(A) 

 ^ Mammae present M 



Symbols (on the right) denote the characters. When a symbol is 

 enclosed in brackets, this means that the structure and functioning of 

 the structure has changed — thus : 



E = Endostyle ; (E) means endostyle that has become thyroid gland. 

 Br = Branchiae ; (Br) = branchial arches that become parts of the 



hyoid or jaw skeletons, etc. 

 But, being the same in development E and (E) ; Br and (Br), etc., are 

 pairs of homologous structures. 



We can now make the formal classification according to the 

 degrees of generality of tectonic structures. 



^Acrania 



Chordata 



\Craniatai 



Cvclostomata 



( Tunicates 

 Amphioxus 



(etc. 



j Lampreys 



i Hagfishes . 

 ( Fishes 



Ichthyopsida 



I 



Gnathostomata' 



Sauropsida 

 Mammals 



Amphibia 

 ■ Reptiles 

 ( Birds 



Mammals 



NE 



N(E)C Dn Br 

 (N)(E)C Dn 

 Br A 



(Br) J(A) 



(N) (E)C Dn 

 (Br)J(A) 



(N)(E)C Dn 

 (Br)J(A)M 



In such a classification we have a root and its divergences. The 

 root is represented by the most generalized of all the characters 

 that are considered — the notochord and the pharyngeal organ, 

 the endostyle. As such these organs characterize the lowest 

 chordates — the Acrania. The addition of a cranial skeleton gives 

 the Cyclostomata, where the notochord is still present in the adult 

 condition but where the endostyle begins to undergo change. 

 Then a jaw-skeleton becomes added to the cranial one, which was 

 all the head skeleton in the Cyclostomes, and so we get the 

 Gnathostomata. Thus the jaw-skeleton is less generalized than 

 the cranium which again is less generalized than the notochordal 

 apparatus — and so on — the same method is applied in all the 

 details of classification of the terminal groups of the scheme of 

 p. 285, that is the Tunicates, Amphioxus, the Cyclostomes, etc. 



