278 THE HISTOEY OF CHEATION. 



remote period in which life on our planet began with the 

 spontaneous generation of Monera, organisms of all groups, 

 both collectively as well as individually, have continually 

 become more perfectly and highly developed. The steadily 

 increasing variety of living forms has always been accom- 

 panied by progress in organization. The lower the strata 

 of the earth in which the remains of extinct animals and 

 plants lie buried, that is, the older the strata are, the more 

 simple and imperfect are the forms which they contain. This 

 applies to organisms collectively, as well as to every single 

 large or small group of them, setting aside, of course, those 

 exceptions which are due to the process of degeneration, 

 which we shall discuss hereafter. 



As a confirmation of this law I shall mention only the 

 most important of all animal groups, the tribe of vertebrate 

 animals. The oldest fossil remains of vertebrate animals 

 known to us belong to the lowest class, that of Fishes. Upon 

 these there followed later more perfect Amphibious animals, 

 then Eeptiles, and lastly, at a much later period, the most 

 highly organized classes of vertebrate animals. Birds and 

 Mammals. Of the latter only the lowest and most imperfect 

 forms, without placenta, appeared at first, such as are the 

 pouched animals (Marsupials), and afterwards, at a much 

 later period, the more perfect mammals, with placenta. Of 

 these, also, at first only the lower kinds appeared, the higher 

 forms later ; and not until the late tertiary period did man 

 gradually develop out of these last. 



If we follow the historical development of the vegetable 

 kingdom we shall find the same law operative there. Of 

 plants there existed at first only the lowest and most im- 

 perfect classes, the Alg?e or tangles. Later there followed 



