June 20, 1902.] 



SCIENCE. 



979 



elements and organs on aceonnt of their 

 positions in space, and these spacial rela- 

 tions are connected with certain fundamen- 

 tal processes of life, viz., the position of the 

 receptive and excretory poles. Type is 

 thus entirely different and distinct from 

 degrees of development. The same typs 

 may be exhibited in several different de- 

 grees" of development, and conversely the 

 same degree of development may be 

 reached in several different types. The re- 

 sult of a degree of development and the 

 type gives the distinguishing character- 

 istics of a class. 



THE DOCTRINE OF TYPES. 



According to this doctrine, the animal 

 •world presents four fundamental types of 

 organization, the peripheral or radiating 

 type, found in infusorians, medusa?, and 

 asteroids; the segmented or length type, 

 found in worms; the massive type, found 

 . in molluscs, and in some radiolarians and 

 infusorians; and finally the vertebrate 

 type, a composite, in which all types are 

 united. Thus the vertebrate brain is built 

 probably after the asteroid type; the vis- 

 cera are certainly molluscan, and the verte- 

 bral column, without doubt, worm-like, 

 though according to the argument in other 

 places, distinctively vertebrate at the same 

 time. 



These four fundamental types are 

 capable, by suppression and expansion, of 

 many combinations, and the amount of 

 suppression or preponderance of the dif- 

 ferent types determines classes, genera, 

 and species. ' If it be true, ' von Baer says, 

 'that the larger and smaller groups of 

 animals depend on this twofold relation, 

 between the degrees of development and 

 the types of organization, then the opinion 

 that there exists an uninterrupted advance 

 from the lower to the higher is based on 

 misconception. ' 



IV. 



APPLICATION OF THE ABOVE DOCTRINE TO THE 

 HISTORY OF INDIVIDUAL DEVELOPMENT. 



(a) It is clear that a higher or a lower 

 degree of development is the same thing as 

 a greater or less degree of diversity in tis- 

 sues and in form. The mass out of which 

 an embryo is molded, and the body mass 

 of the simplest animal, are very much alike, 

 for in both there is little distinctness of 

 form, and slight contrast of parts. If 

 therefore we discover in the tissues of some 

 lower animals a greater degree of diversity 

 than in others, and place them in series 

 according to the differences presented, we 

 find many coincidences between the ob- 

 served facts and the requirements of the 

 genetic law implied by this series. 



(b) These coincidences between the 

 facts and the theoretical requirements, 

 however, do not show that the embryo of 

 a higher form passes gradually through the 

 adult stages of lower ones. It seems, in 

 fact, as though the type of each animal 

 were immediately impressed upon its em- 

 bryo, and that this governs its whole de- 

 velopment. The history of the chick is a 

 commentary to this statement. 



The first organs to be distinguished in 

 the germ are those of the vertebrate type, 

 and it is clear that after their appearance 

 resemblance to an invertebrate can no 

 longer be held. In the beginning of their 

 development, all classes of vertebrates are 

 very similar, and so we can say that the 

 embryo of a vertebrate is from the begin- 

 ning a vertebrate and has at no time any 

 resemblance to an invertebrate. An adult 

 animal, having the vertebrate type and 

 such slight diversity of tissues and dis- 

 tinctness of form as the vertebrate embryo, 

 is unknown, and so the embryos of verte- 

 brates in their development do not pass 

 through the adult stages of any known 

 animals. 



