HISTORY OF THE PLANT KINGDOM 299 



as in zoology, it is a well-recognized law that the develop- 

 ment of every individual usually follows in outline the course 

 of development of its group. During the process of hatch- 

 ing, while the young animal in the egg is beginning to 

 develop into a turtle, an alligator, or a bird, the general 

 form of the embryo is for some time much the same in 

 all three cases. It is probable that this arises from the 

 fact that turtles, alligators, and birds have sprung from 

 a common ancestor, — an animal which lived in the far 

 remote past and which united in its organization some of 

 the characteristics of these its descendants. 



Reasoning in this same way, we may, for example, feel 

 sure from the resemblance in essentials between the pro- 

 thallia of ferns and horsetails that these two kinds of 

 plants, so different in the general form and structure of 

 the full-grown sporophytes, have a common ancestry. 

 This is only one rather simple instance, out of many, 

 of likeness in the early stages in the life history of two 

 classes of plants which are most unlike in their adult con- 

 dition. By comparing in this way the successive steps in 

 the development of great numbers of plants of different 

 groups, it has become possible to draw up a sort of pedi- 

 gree of the plant world. This is not as yet by any means 

 complete, but what is already known on the subject throws 

 much light on the reasons for the existence of structures 

 in the early part of the life histories of many plants which 

 would otherwise seem to be wholly useless and without 

 meaning.^ 



380. Plants form an Ascending Series. — All modern 

 systems of classification group plants in such a way as 

 to show a succession of steps, often irregular and broken, 



1 See Bergen and Davis' Principles of Botany, pp. 243, 273, 404, 405. 



