452 SKETCH OF PALEOBOTANY. 



epochs is also indicated. The only exception to this rule has been made 

 in the case of the Cellular Cryptogams, whose heterogeneous character 

 has doubtless caused it to undergo considerable fluctuation. One such 

 is assumed in the Carboniferous, in which, though one of the great pe- 

 riods of vegetable deposition, the actual number of Cellular Cryptogams 

 falls below that of either preceding or subsequent periods. This seems 

 to argue that there was a reduced representation of this form of plant 

 life in that age, and this is shown in the figure presented for that type. 



The three facts which this diagram aims chiefly to bring out, not 

 shown in either of the preceding diagrams, are, first, the true origin, or 

 geological age of first appearauce of each type of vegetation ; second, 

 the period of its maximum development ; and, third, the rank it occu- 

 pies in the living flora relative to its maximum. These are all delicate 

 points to fix in a manner that will satisfy all the conditions of the problem. 

 The evidence from all sides has to be cautiously weighed, care taken not 

 to give undue weight to any nor to undervalue anj-. These are not ques- 

 tions that can be hastily settled. They require to be pondered long 

 and well. It is by no means claimed that substantial truth has been 

 reached in every case. No two persons, however competent, would 

 probably exactly agree upon all the points, and I am sure that at differ- 

 ent times with increasing evidence I have modified my own conclusions. 

 But this is far from confessing that the attempt is valueless, and it is 

 certain that great value should be attached to the enlarged conceptions 

 of vegetal development that flow from such a study. 



Descent of plants. — But we need not stop here. The great law of de- 

 velopment does not allow us to contemijlate these types as independent 

 of one another. Each class of plants must be regarded as the descend- 

 ants of some ancestral form more or less different from it. The multiple 

 origin of existing forms, whether of plants or animalSj is repugnant to 

 modern scientific thought. It is the discovery of facts that has rendered 

 it so. The multiple and varied of the present must be regarded as due 

 to divergences in the past. The forms we have have come down to us 

 along divergent lines from common ancestral forms. These are the 

 lines of descent, and plants have their lines of descent as well as animals 

 or human families. Of this we are practically certain, but just what 

 those lines are and where they diverged — these are the great problems 

 of phytogeny. 



The lines of descent in the animal kingdom have been laid down by 

 various eminent zoologists with considerable confidence and unanimity. 

 In plant life they have scarcely ever been attempted. The problem is 

 loaded with extraordinary complications and cannot be satisfactorily 

 attacked until we shall possess far more knowledge than we possess at 

 present. 



