180 K. E. VON BAER. PHILOSOPillCAL FRAGMENTS. 



bird's beak must be more in harmony with a bird's body and 

 more easily combined with it, than with a mammalian body, and 

 still less with a frog's body ; for with a multitude of beaked 

 birds, we have only a couple of beaked mammalia, and no beaked 

 frog at all. Here we have, not to interpret, but to conceive 

 what actually exists. 



We see in fact, that certain principal models of the animal 

 kingdom become modified among various animals, not however 

 equally, but in such a manner that the majority of these modi- 

 fications are more closely allied than the rest. The principal 

 models may be called Provinces of the animal kingdom, and their 

 most important modifications, Classes ; these being again mo- 

 dified into subordinate forms, Families, and so forth. It happens 

 always, however, that among the subordinate modifications of 

 any degree, the majority resemble one another, and only a 

 few diverge ; so that the organic theme with its variations may 

 be compared to a sphere composed of a densely compressed 

 centre and a much more thinly populated atmosphere. Upon 

 the same relations again depend what we call genus and species. 

 The species consists of a number of individuals, which are indeed 

 not perfectly similar, but most of which exhibit an obvious 

 similarity ; whilst a few are often so aberrant, that it might be 

 seriously doubted whether they are to be grouped under this 

 form or not. These aberrations, however, are not only rare, but 

 Nature affords them less support, so that they continually tend, 

 either in themselves or in their progeny, to return to the normal 

 standard. There are, further, two remarkable laws : the first, 

 that the more dense the centre, the less extensive is the atmo- 

 sphere, as well in the larger spheres as in the smaller subordinate 

 ones. So even in the species. If the individuals which may be 

 unquestionably referred to one species, yet differ a good deal 

 from one another, then the number oP those concerning which 

 well-founded doubts may be entertained, is not inconsiderable. 

 The more similar, on the other hand, the normal individuals are 

 to one another, so much the less frequently, or even not at all, 

 do the transitional forms occur. Secondly, in every larger 

 sphere the subordinate spheres of the centre are richer in se- 

 condary forms than the sphere of the periphery. In fact, if we 

 take single species, we find that they are the more rich in indi- 



