THE EVOLUTION OF PLANTS. 607 



groups, within the limits of each of which all stages of complexity in reproduction 

 occur. The attempt to string together forms agreeing in reproductive methods 

 makes it necessary to break up groups which on general grounds seem to be natural 

 families. And as it is a natural system that we are striving after, systems like that 

 of Sachs (which may be compared to the artificial sexual system of Linnseus) must 

 be abandoned. That the publication of the Sachsian system in his widely-read 

 "Text-book" has done great service to Botany there can be no doubt; it has stimu- 

 lated thought and observation, and has led more speedily than would otherwise 

 have been the case to the establishment of broad and probably sound views as to 

 the relations of the Thallophytes. However, the Myxomycetes, approaching as they 

 do certain groups of the animal kingdom, are kept apart from the rest of the 

 Thallophytes in the most recent system. 



The classification of plants according to their similarity of structure — species 

 into genera, genera into families or orders, families into alliances or cohorts, these 

 into classes, and classes into two chief branches or phyla, the Cryptogams and 

 Phanerogams — leads to the presumption that these two chief branches have arisen 

 from a common stock, have diverged from a common stem. A consideration of all 

 animal and plant forms similarly leads us to the belief that the main stems of 

 the Animal and Vegetable Kingdoms, respectively, meet at their points of origin. 

 By studying systems of classification drawn up on paper and restricted to two 

 dimensions of space, we involuntarily conceive the classes and orders of the vegetable 

 kingdom, as a tree which continually branches, finally ending in thousands of twigs 

 which represent the various species. Such is, rightly or wrongly, the conception of 

 all Botanists who have concerned themselves with the construction of a natural 

 system. They only difier in so far that some regard the Thallophytes as standing 

 at the base, and derive from these the Liverworts and Mosses, from these the 

 Ferns, &c., and so on to the Gymnosperms and Angiosperms; whilst others make a 

 subdivision of the main trunk at once into Cryptogams and Phanerogams, each of 

 these continually branching according to the various classes and families. Others 

 again, whilst conceiving the whole vegetable kingdom as having a common origin, 

 regard this as the centre of a sphere, and that the several phyla and classes radiate 

 out from this, producing numerous branches and twigs at the surface of the sphere. 

 Each of these hypotheses presupposes, in the first instance, the existence (or spon- 

 taneous generation) of a few Thallophytes of extremely simple structure which have 

 become differentiated, i.e. given rise to more complex offspring which form the 

 beginnings of the branches of the tree. To this kind of development of a tree- 

 structure, the terms Phylogenesis or Phylogeny (from (pv\ii, a tribe; and yei^^da, 

 to produce) is given. Obviously, not only the original forms possess the capacity 

 of differentiating, but their offspring also, and so on through the entire tree. But 

 views are divided ac to whether this continued differentiation follows a predeter- 

 mined plan, is due to definite inherent forces, or whether it may not be restricted in 

 this sense and due to other and external causes. 



