102 ESSAY ON CLASSIFICATION. 



It was indeed a gigantic step to demonstrate such an 

 identity in the material basis of the development of all 

 animals, when their anatomical structure was already 

 known to exhibit such radically different plans in their 

 full-grown state. From that time a more and more ex- 

 tensive investigation of the manner in which the first germ 

 is formed in these eggs, and the embryo developes itself ; 

 how its organs grow gradually out of a homogeneous mass ; 

 what changes, what complications, what connections, what 

 functions, they exhibit at every stage ; how, in the end, 

 the young animal assumes its final form and structure, 

 and becomes a new, independent being, could not fail to 

 be a most interesting subject of inquiry. To ascertain 

 all this, in as many animals as possible, belonging to the 

 most different types of the animal kingdom, soon became 

 the principal aim of all embryological investigations ; and 

 it can truly be said, that few sciences have advanced with 

 such astonishing rapidity, or led to more satisfactory 

 results. 



For the actual phases of the mode of development of 

 the different types of the animal kingdom, I must refer to 

 special works upon this subject, 1 no general treatise, eni- 



VALENCIENNES (A.) and FREMY, Re- Batrachians, the Lepidosteus, the 

 cherches sur la composition des oeufs Arnia, etc.; not to speak of the op- 

 dans la serie des animaux, Compt.- portuuitics which thousands of miles 

 Rend., 1854, vol. 39, p. 469, 525 and of sea-coast, everywhere easily acces- 

 570. sible, aftbrd for embryological inves- 

 1 The limited attention thus far tigations, from the borders of the 

 paid in this country to the study of Arctics to the Tropics. In connexion 

 Embryology, has induced me to enu- with Embryology, the question of 

 merate the works relating to this Individuality comes up naturally, 

 branch of science more fully than any See upon this subject: LEUCKART 

 others, in the hope of stimulating in- (RuD.), Ueber den Polymorphismus 

 vestigations in this direction. There der Individuen oder die Erscheinung 

 exist, upon this continent, a number der Arbeitstheilung in der Natur; 

 of types of animals, the embryologi- Giessen, 1851, 4to. REICHERT (C. 

 cal illustration of which would add B.), Die monogene Fortpflanzung; 

 immensely to the stock of our science: Dorpat, 1852. HUXLEY (Tn. II.), 

 such are the Opossum, the Ichthyoid Upon Animal Individuality ; Ann. 



