6 DR. HICKS, ON FRESH-WATER ALGA, ETC. 
ence of Palmoglea Brebissonii by its form alone? At the 
same time I am not disputing the separate existence of these 
two plants. If, however, it could be certainly shown that 
the cladonia form never conjugated, then it must be admitted 
that they were essentially distinct; but im this case our 
knowledge is arrived at by the study of the life-history and 
not by the appearance. ‘The form of the cells, and the dis- 
position of the chlorophyl in Palmoglwa Brebissonu through- 
out the mass at the period of conjugation, varies much, some 
of the cells being scarcely oval, while the length of others 
exceeds three or four times their breadth. Mr. Archer also 
is disposed to think that I confound chlorophyle-bearmg 
plants with those having phycophyle; and as a ground for 
this, he cannot conceive of one being produced by the other. 
In answer to this I may say that there are many instances to 
be found; of their mutual exchange so much so as to do 
away with the value of this distinction of colour. Instances 
may be found repeatedly in Collema, particularly during the 
segmentation of the gonidia, and their change into nostoc ; 
the transition is gradual in the various periods from the true 
chlorophyl to the phycophyle. A distinct change from bright 
green to leaden blue may be observed during the segmenta- 
tion of the gonidia of the confervoid filaments on barks of 
trees, as already slightly alluded to in my paper in ‘ Linnean 
Trans.’ on this subject. Many other instances of this occur, 
so that it is impossible to lay much stress on this point. 
Again, Mr. Archer thinks that the maintenance of the 
characters in these simple forms in diverse circumstances and 
places as a proof of the fixity of a large number of species 
which he quotes, and has a difficulty in imagining how a 
Lichen or Moss-gonidium can readily be conceived to change 
now into one form, and now into another. But nothing is 
easier to conceive than that simple gonidia from many 
sources do divide and grow into these forms; that it is the 
ordinary law of their growth; and that many of these forms, 
apparently somewhat dissimilar in external form and internal 
arrangement, can and do spring from the same source, and 
that it is their ordinary mode of so doing. If this be received 
the rest is simply a matter of observation. If the life-history 
shows it, we are bound to accept it, whether according to 
preconception or not. And it must not be forgotten that 
my observations extended only to a few species, and yet in 
them there is considerable variation in form and shape. It 
must also be noted that I do not say all the Palmellacee and 
kindred forms arise from Cladonia. I only show a great 
many do, and also that similar forms spring from mosses 
