INTRODUCTION 55 



probable that the chain of plant-forms is a linear 

 series. Some of the lower types have become more 

 specialised and have then diverged or branched off 

 from others which have given rise to a still higher 

 group, and from these again other groups have 

 diverged and proceeded no further, whilst in other 

 directions their common ancestors have given rise in 

 different directions to higher forms in a progressive 

 series. The relationship between the different groups 

 of plants may thus be compared with the ramifica- 

 tions of the branches of a tree. 



The origin of the higher plants from the Bryo- 

 phytes,* following Campbell, t is shown on page 56. 

 In the Bryophytes archegonia and antheridia are 

 developed. These organs are found on the gameto- 

 phyte or sexual generation, as in the higher 

 Cryptogams and Phanerogams. 



A consideration of such a scheme or phylogenetic 

 tree, which graphically indicates the probable history 

 of each group, shows that there are several features 

 in the evolution of plants that regulate their history 

 in time and space. 



In the first place the branches of this tree show 

 that in the main groups of plants there has been 

 considerable variation in the original forms since 

 they were first produced, otherwise we should have 

 no divergence. 



If one considers the vast number of individuals 



* On this point the researches of Dr. F. Cavers should be con- 

 sulted, 

 t Cf. also Bessey, Moss, Tansley, Wernham, and others. 



