78 ESSAY ON CLASSIFICATION 



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

 be the 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, became soon the principal aim of all embryo- 

 logical investigations; and it can truly be said that few sciences have 

 advanced with such astonishing rapidity and 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 the special works upon 

 this subject,^*'* no general treatise embracing the most recent investi- 

 gations having as yet been published; and I must take it for granted 

 that before forming a definite opinion upon the comparisons insti- 

 tuted hereafter between the growth of animals and the structural 

 gradation among full-grown animals, or the order of succession of 

 the fossils characteristic of different geological periods, the necessary 

 information respecting these changes has been gathered by my 

 readers and sufficiently mastered to enable them to deal with it 

 freely. 



The embryology of Polypi has been very little studied thus far; 

 what we know of the embryonic growth of these animals relates 

 chiefly to the family of Actinoids. Whep the young is hatched, it has 

 the form of a little club-shaped or pear-shaped body, which soon 

 assumes the appearance of the adult, from which it differs only by 

 having few tentacles. The mode of ramification and the multiplica- 

 tion by buds have, however, been carefully and minutely studied in 

 all the families of this class. Acalephs present phenomena so pe- 

 culiar that they are discussed hereafter in a special section. Their 



^°*The limited attention thus far paid in this country to the study of Embryology 

 has induced me to enumerate the works relating to this branch of science more 

 fully than any others, in the hope of stimulating investigations in this direction. 

 There exist upon this continent a number of types of animals, the embryological 

 illustration of which would add immensely to the stock of our science; such are the 

 Opossum, the Ichthyoid Batrachians, the Leipidosteus, the Amia, etc., not to speak of 

 the opportunities which thousands of miles of sea-coast, everywhere easily accessible, 

 afford for embryological investigations, from the borders of the Arctics to the Tropics. 

 In connection with Embryology the question of Individuality comes up naturally. See 

 upon this subject: Rudolf Leuckart, Ueber den Polymorphistnus dcr Individuen oder 

 die Erscheinung der Arbeitscheilung in der Natur (Giessen, 1851); Thomas Henry 

 Huxley, "Upon Animal Individuality," Annals and Magazine of Natural History, IX 

 (2d ser., 1852), 507; Edward Forbes, "On the supposed Analogy between the Life of an 

 Individual and the Duration of a Species," ibid., X (2d ser., 1852), 59; Alexander Braun, 

 Das Individuum der P flame (Berlin, 1853), and Betrachtungen iiber die Erscheinung 

 der VerjiiJigung in der Natur (Freiburg, 1819). 



