The Principles of Morphology. 
125 
uninfluenced by prejudiced conceptions of the phylogeny of any 
particular group. I believe also that teratological phenomena are 
in this direction very helpful, for in these strictly-limited mani¬ 
festations of morphological law, we find the organs assuming forms, 
the replicas of which will be found for the searching in other 
departments of vegetable life. The facts of individual development 
will also sometimes be of use, but very often also misleading, on 
account of the fact that many of the recapitulative stages derived 
from the race-history are missed out owing to the abbreviated 
course taken by the ontogeny. 
It is, then, owing to the stereotyped fixity of the three 
categories of organs, whereby no obliteration of the distinctive 
boundaries of each, i.e. no fusion or confusion of any two 
categories, ever takes place, that a definite science of morphology 
becomes possible. Each type of organ, whether caulome, phyllome 
or rhizome, has evolved, through infinite variation, along its own 
line, diverging away from each of the other types, precisely in the 
same way as genera and species of plants have evolved into 
distinctive entities. In any particular case, where it is especially 
difficult or impossible to determine the morphological nature of an 
organ, this must be attributed solely to our own ignorance and 
inability to trace all the stages in modification which the particular 
organ in question has undergone during the period of its evolution. 
The Origin of the Vascular Plant. 
I. The Alternation of Generations. 
HE vegetable kingdom, doubtless, was at one time constituted 
1 solely of unicellular organisms which eventually gave rise by 
successive cell-divisions both to the filamentous and lamellifonn 
types. Filamentous aggregations, forming so-called individual 
plants , were produced by branching of these filaments; yet multi¬ 
plication of the thallus also eventually occurred by means of tissue- 
formation, either in the form of a true tissue, as in Liverworts and 
Mosses, or of a false tissue, such as that of Seaweeds and Fungi. 
The thallus of these early plant-types assumes various forms; it is 
either dorsiventral, as in Liverworts; cylindrical, asintheMushroom- 
stalk and prothallus of some Lycopods; partly cylindrical and partly 
isobilateral, as in Brown Seaweeds, or filamentous, as in many Algae, 
Fungi, Prothalli, and the Moss-protonema. 
