172 WHAT IS LIFE ? 



But all animals, all vegetables, even Man himself, were 

 and are built up of cells and the secretions of cells- 

 successive periods of tliis earth's history, a continual increase in the 

 perfection of organic formations has taken place. Since that incon- 

 ceivably 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 per- 

 fectly and highly developed. The steadily increasing variety of living 

 forms has always been accompanied 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 Reptiles, 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 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 imperfect classes, the Algos or tangles. 

 Later there followed the group of Ferns or Filicinse (ferns, pole-reeds, 

 scale-plants, etc.). But as yet there existed no flowering plants, or 

 Phanerogama. These originated later with the Gymnosperms (firs 

 and cycads), whose whole structure stands far below that of the 

 other flowering plants ( Angiosperms) , and forms the transition from 

 the group of fern-like plants to the Angiosperms. These latter 

 developed at a still later date, and among them there were at first only 

 flowering plants without corolla (Monocotyledons andMonochlamyds); 



