REVIEW AND SUMMARY. 265 



other cellular cryptogams were the dominant forms of 

 plant life. Vascular cryptogams appeared in the Devo- 

 nian ; after them came the gymnosperms ; then the mono- 

 cotyledons ; and finally the different classes of dicotyledons 

 attained their present supremacy. 



The life history of the flowering plants and higher cryp- 

 togams still further confirms the same view, passing as 

 they do through successive stages of development that 

 repeat in miniature the history of past ages of plant life. 

 The fern prothallium in its earlier stages of growth is so 

 nearly a filamentous green alga as to be distinguished 

 from one by its origin rather than by its structure ; a little 

 later it becomes a flat expansion of cells, so like a liver- 

 wort as to deceive the inexperienced eye ; and these and 

 other phases of their developmental history may still be 

 recognized, not only in the gymnosperms, but in the higher 

 flowering plants. 



From facts like these, it seems impossible to draw any 

 other conclusion than that there has been from the earliest 

 appearance of plant life on the globe a slowly 



,, ,. ■ 1,1-1 Conclusions. 



progressive development from simpler to higher 

 forms, and that the record of this is still preserved to us 

 in the natural groups that form the present vegetation of 

 the earth. 



We are to think, then, of the plants we have studied 

 and those we have yet to study, as in reality all members of 

 one vast and ancient family, some closely, others remotely 

 related, some still retaining the simple forms and habits of 

 earlier days, and others, through a long course of selection, 

 variously modified to meet external conditions and often 

 exquisitely adapted to animal structures no less highly 

 modified and adapted to them. In this great family, we 



