-56 General Part. 



This grouping of animals in grades is, as noticed above, not 

 ■arbitrary but founded on Nature itself. The animals are connected 

 by some degree of relationship, i.e., by a more or less exact 

 correspondence of structure : this connection finds an incomplete 

 •expression in the classification of the Animal Kingdom. When, 

 for instance, Fish, Amphibia, Eeptiles, Birds, and Mammals are 

 comprised in one group, it is indeed implied, with sufficient clearness, 

 that all these divisions agree in certain important points of structure, 

 but not that they are united together like the links of a chain. 

 Such is, however, the case, and Amphibia follow Pisces, Eeptilia, 

 .Amphibia ; Aves and Mammalia, the Eeptilia. Such a connection 

 of forms, more intimate than the system expresses, exists in the 

 Animal Kingdom; a true linking together, such as that just set 

 .forth for the principal groups of the Vertebrata is everywhere 

 more or less easily recognisable. 



What remains now is to ask the reason of this remarkable connection 

 of different animals. Until a few years ago, the answer always was 

 that the whole situation is one of the enigmas of Nature, incom- 

 prehensible to the mind of man. Now it is universally acknowledged 

 that this affinity, this similarity, of different animals is an effect of 

 those laws which govern the resemblance between parent and child, 

 brother and sister, or more distant relations, viz., the laws of 

 heredity. The lion, the tiger, the wildcat and other species, in 

 virtue of a close accordance in most points of structure, all belong 

 to one genus, implying that they all originally sprang from one 

 species, which has gradually broken up into several. If bears, 

 martens, cats, etc., are all united in one order, it is because 

 they have all sprung from a common ancestor; so also for 

 the larger groups. Mammalia, Yertebrata. Two such groups 

 as Eeptiles and Birds are linked together, because the latter have 

 arisen from the former : by gradual change, a branch from the 

 Eeptile-stem is modified towards a Bird-form, from which the rest 

 have sprung. 



The completion of the conception leads to the conclusion that 

 all animals have originated in one common primitive ancestor, 

 which was probably similar to the Amoeba. This is the purport of the 

 Theory of Descent (Darwinism), according to which all the 

 immense multiplicity of plants and animals have gradually evolved, 

 in the course of enormous periods of time, from the same progenitor. 



That this theory is correct is evident on the one hand, because it 

 not only elucidates the long-noticed " conformity to type," in a natural 

 way, but also renders intelligible an endless number of other 

 phenomena of the organic world ; and on the other, because in spite 

 •of strenuous efforts, facts irreconcilable with the theory have not 

 been found. For the most important points supporting the theory of 

 descent, without the acceptance of which, indeed, they are incompre- 



