264 STUDY OF COMMOK PLANTS. 



factory as far as it goes, is given by Miiller in his general 

 retrospect at the close of the Fertilization of Flowers, as 

 follows : " Insects must operate by selection in the same 

 way as do unscientific cultivators among men, who preserve 

 the most pleasing or most useful specimens, and reject or 

 neglect the others. In both cases, selection in course of 

 time brings those variations to perfection which corre- 

 spond to the taste or to the needs of the selective agent. 

 Different groups of insects, according to their sense of 

 taste or color, the length of their tongues, their way of 

 movement and their dexterity, have produced various 

 odors, colors, and forms of flowers; and insects and flowers 

 have progressed together towards perfection." 



Turning to the lower or so-called cryptogamic plants, it 

 appears that precisely the same principles hold good. Ferns 

 and mosses, quite as plainly as plants higher in 

 A progressiTe the scale, exhibit degrees of relationship. Here, 

 series. ^^ elsewhere, closely related .species fall natu- 



rally into genera, closely related genera into families, and 

 these into orders and higher groups. Furthermore, a 

 review of these higher groups shows that the vegetable 

 kingdom as it exists to-day presents a progressive series, 

 rising from such simple plants as Spirogyra, and even more 

 primitive forms of the green algse, through the liverworts 

 and mosses to the vascular cryptogams, and from these by 

 an almost insensible step through Selaginella and its allies 

 up to the gymnosperms and flowering plants. It is be- 

 lieved by those who have the most extended and critical 

 knowledge of plant life that this series corresponds closely 

 with the order of development of the vegetable kingdom, 

 and, as a matter of fact, it is found that the geological 

 record strikingly confirms this view. In earlier geological 

 times, beginning with the Silurian Age, marine algse and 



