DIFFERENTIATION OF ORGANS IN SPERMAPHYTA 19 



frequently proceeded from very simple forms as their starting point, and 

 yet that the formation of organs has taken place along these lines in 

 a more or less similar manner because of the similar capacity for develop- 

 ment they have derived from the ' stem form ' ; in other words, the 

 material nature being similar the development must proceed along 

 a similar path. As an example of this we may cite the homologous 

 sporogonia of the mosses and liverworts. In them we have two series 

 which must have branched off the one from the other when the formation 

 of sporangia was as simple as we find it nowadays in Coleochaete l , where 

 indeed we can scarcely speak of sporangia. In both series the further 

 development has gone to a considerable length, but whilst the essentials 

 of the sporangia, which lie only in the function of producing spores, have 

 been retained in both, the relationships of configuration otherwise have 

 diverged in widely different directions. 



The vegetative organs of the liverworts furnish even a better 

 illustration of this. If we assume that there has been a main line of 

 development it must have happened that the leafy shoots of the liverworts 

 have taken origin more tJian once as different series independent of one 

 another. The leaves of the acrogynous and anacrogynous liverworts, which 

 we may take as an example, would then not be homologous ; neverthe- 

 less they arise on the vegetative apex in essentially the same way, and 

 conform so closely with one another in their other features that they are 

 clearly structures which have something in common. It often happens 

 that with such examples before us we speak of a homology of organization 

 which is really not phylogenetic, or at least has only to do with 

 phylogeny in so far as it recognizes a common capacity for development 

 derivable from undififerentiated ancestors. Such a conception is a more 

 complex one than that involved in the ordinary and usually somewhat 

 speculative phylogenetic definitions, but it fits the facts better. 



Analogous structures, by which we mean organs which are alike only 

 in their ' adaptation ' to the outer world, and this, it must be remembered, 

 may be achieved in quite different ways, should be distinguished from 

 those which are homologous. I need only here recall, in illustration, the 

 occurrence of Euphorbiaceae with the habit of Cacteae ; the many plants 

 possessing needle-leaves and belonging to different families ; the occurrence 

 of a porose capsule in Polytrichum as well as in Papaver ; the appearance 

 of elaters quite similar to those of many liverworts in Battarea, one of 

 the Fungi. It is facts like these which have led to the belief that 

 morphology has no concern with the function of organs, because other- 

 wise homologous and analogous organs might be confused ; but we have 



1 I do not regard Coleochaete as an ' archetype ' of the Archegoniatae, and I mention it here 

 merely for the sake of a comparison. 



C 2 



