500 



SCIENCE 



[N. S. Vol. XXVI. No. 608 



without rigorous attention to the applica- 

 bility of the characters assigned. Much 

 better was the work of the greatest nat- 

 uralist of all, Johannes Miiller, who ad- 

 vanced our knowledge of the systematic 

 relations of all classes of vertebrates as 

 well as invertebrates. But all were unable 

 to free themselves from the incubus of the 

 popular idea that all branchiferous verte- 

 brates formed a unit to be compared with 

 birds and mammals. Several propositions 

 to segregate, as classes, Amphioxus and 

 the chondropterygians had been made, and 

 Louis Agassiz deserves the credit of claim- 

 ing class value for the myzonts or marsipo- 

 branchs as well as the selachians. But it 

 was left to Ernst Haeckel, a pupil of Miil- 

 ler, still happily living, to divest himself 

 entirely of ancient prejudices and appre- 

 ciate the interrelationship of the primary 

 sections of the vertebrate branch. He for 

 the first time (1866) set apart the amphi- 

 oxids in a group opposed to all other verte- 

 brates, then docked off the marsipobranchs 

 from all the rest, and collected the classes 

 generally recognized in essentially the same 

 manner as is now prevalent. We may 

 differ from Haeckel as to his classes of 

 fishes and dipnoans, but his correctness in 

 the action just noticed will be conceded by 

 most, if not all, systematic zoologists to-day. 



EMBRYOLOGY 



While Cuvier was still flourishing, a 

 school of investigators into the develop- 

 mental changes of the individual in dif- 

 ferent classes, and among them the verte- 

 brates," was accumulating new material 

 which should be of use to the systematic 

 zoologist. Chief of these was Karl Ernst 

 von Baer. In various memoirs (1826 et 

 seq.) he subjected the major classification 

 of animals to a critical review from an 

 embryological point of view, recognized, 

 with Cuvier, the existence of four distinct 

 plans which he called types and charac- 



terized them in embryological terms— S-yo- 

 lutio radiata, Evolutio contorta (molluscs), 

 Evolutio gemina (articulates) and Evolu- 

 tio higemina (vertebrates). The last were 

 successively differentiated on account of 

 the embryonic changes from the fishes to 

 the mammals. "These Beitrage, " Louis 

 Agassiz justly affirmed, "and the papers 

 in which Cuvier characterized for the first 

 time the four great types of the animal 

 kingdom, are among the most important 

 contributions to general zoology ever pub- 

 lished." 



One of the most notable results, so 

 far as systematic zoology was involved, 

 was the deduction forced on Kowalevsky 

 by his investigation of the embryology of 

 tunicates, that those animals, long asso- 

 ciated with acephalous moUusks, were 

 really degenerate and specialized protover- 

 tebrates. This view early won general ac- 

 ceptance. 



While embryology was very successfully 

 used for the elucidation of systematic zool- 

 ogy its facts were often misunderstood and 

 perverted. For instance, the cetaceans 

 were regarded as low because they had a 

 primitive fish-like form, although it must 

 be obvious to all logical zoologists of the 

 present time that they are derived from a 

 quadruped stock; snakes have been also 

 regarded as inferior in the scale becaiise 

 no legs were developed, although it would 

 be now conceded by every instructed her- 

 petologist that they are descendants of 

 footed or lizard-like reptiles. Ainmocoetes 

 was considered as higher than Petromyzon 

 "inasmuch as the division of the lips indi- 

 cates a tendency towards a formation of a 

 distinct upper and lower jaw," but we 

 now know that AmmocoBtes is the larval 

 form of Petromyzon. Innumerable still 

 more pertinent examples might be adduced 

 for the inferior systematic grades, orders, 

 families, genera, species, etc. The words 

 high and low were used when generalized 



