eS 
Oct. 26, 1871 | 
NATURE 
507 

In a large proportion of the essays I have written, the same sub- 
ject of plane position has had to be considered and described I 
am, therefore, somewhat seriously interested in opposing as well 
the disuse of the word “ position,” which no one can misunder- 
stand, as the use of the words ‘‘aspect,” ‘‘slope,” ‘‘tilt,” &c, 
in a sense not at present assigned (nor properly assignable) to 
them, RicHp. A, Proctor 

Sea-water Aquaria 
I HAVE read with much gusto your article upon the Crystal 
Palace Aquarium, Iam induced by it to put forward a caution 
with regard to the construction of rock-work in tanks. 
Several weeks ago, casually looking over a heap of Bangor 
slaty rock, on the road bordering the Brighton Aquarium works, 
and being used for the rock-work of tanks, my attention was 
attracted by some bright green patches upon some of the 
stones, which appeared to me to be carbonate of copper, but 
was probably silicate. Looking further at one with a lens, 
I imagined that I could also distinguish particles of pea- 
cock ore. On attempting to purloin a specimen, I was 
very properly stopped from so criminal an act by the Cerberus 
in charge. I wrote to the chairman of the company, stating 
that, not having examined the stone, I might be only contributing | 
a mare’s nest to their zoological collection, but that if it con- 
tained much ‘copper the fish would be in danger. I understand 
that upon receipt of my letter some rock was sent up to Dr. 
much sulphide of copper, and that the pretty green rock was 
therefore unfit for tank rock-work. 
I think this will serve as a caution to the constructors of aquaria 
to examine all material which is to be in contact with water most | 
carefully before using it. There are so many minerals which 
would be deleterious that I strongly advise an analysis and report 
in the case of every untried rock. The accident of my passing a 
heap of stones has saved the company, with whichI am not in 
the least connected except as a fervent well-wisher, from a large 
expenditure and a serious scrape. 
Allow me to ask those who are accustomed to the manage- 
ment of tanks, whether hydraulic pressure upon a small and 
strong one would be likely to assist in maintaining life in any of 
the deep-sea organisms, and whether it would be useful to make 
recesses for those loving darkness, with the axes opposite the 
plate glass side, so that a bull’s-eye lantern could occasionally 
throw light upon their actions and mode of life ? 
Brighton, Oct. 21 MARSHALL HALL 
ON HOMOPLASTIC AGREEMENTS IN 
PLANTS 
ye the recent meeting of the British Association I 
pointed out in a short communication the difference 
that existed between mimicry in animals and what has 
been spoken of under that name amongst plants. The 

distinction was sufficiently obvious, and must have oc- | 
curred to everyone who had given the matter any consi- 
deration, but my object was to try to raise a discussion 
upon the whole subject as exhibited in plants. 
I fancy it is hardly sufficiently understood how com- 
monly this agreement of facies occur in plants widely 
differing in other respects. I will give a few illustrations 
of it. Humboldt remarks (“ Views of Nature,” p. 351) : 
“Tn all European colonies the inhabitants have been led | 
by resemblances of physiognomy (/adztws, facies) to apply 
the names of European forms to certain tropical plants, 
which bear wholly different flowers and fruits from the 
genera to which these designations originally referred. 
Everywhere in both hemispheres the northern settler has 
believed he could recognise alders, poplars, apple, and 
olive trees, being misled for the most part by the form of 
the leaves and the direction of the branches.” Nor has 
the popular eye alone been deceived by these resem- 
blances. Schleidenstates(“The Plant,” p. 255) that Australia 
has in common with Europe a very common plant, the 
daisy, yet Dr. Hooker has pointed out (Flora of Tasmania, 
pl. 47) that the plant intended by Schleiden is the very 
| 

similar but distinct Bvachycoma decipiens Hook. fil. 
Again, true flowering plants belonging to the very curious 
family Podostemacee have been figured as liverworts and 
other cryptogamic plants (Berkeley, Intr. to Crypt. Bot., 
p. 5). Many other instances of similar errors might be 
given.* 
Since I read my paper, I have met with an essay by 
Schouw, in which he enumerates facts of the same kind. 
“ There is still,” he says (‘‘Earth, Plants, and Man,” p. 61), 
“another kind of repetition which I might call habitual 
repetition, or denominate #zémzcry, if this expression was 
not at variance with the subjection to law which exists 
throughout nature, but to comprehend which our powers 
are oiten insufficient.” After various illustrations he pro- 
ceeds :—“In the genus J7u/ista we have the remarkable 
sight of a compositous flower, with the tendrils of a 
leguminous plant.” (This by an accidental coincidence 
was one of the instances which I, myself, used at Edin- 
burgh.) “In Begonia fuchsioides the leaves are similar 
to a Fuchsia, and very different from the other forms of 
leaf among the begonias, and the colour of the blossom 
likewise reminds us of the fuchsias. We have another 
most striking example in certain Brazilian plants, which 
although possessed of perfectly developed flowers and 
| fruits, mimic, as it were, in their leaves and stems, groups 
Percy, whose report, I am told, was to the effect that there was | a 5 ; eee 
of plants of much lower rank.” (He is alluding to the 
Podostemacee mentioned above.) “ Laczs fucotdes re- 
sembles certain seaweeds so much, that it might be mis- 
taken for one by a person who did not see the flowers. 
Mniopsis scaturiginum strikingly resembles a Funger- 
mannia.” 
I suggested that when a plant put on the characteristic 
facies of a distinct natural family, it might conveniently 
be spoken of as a pseudomorph, having in view an obvious 
analogy in the case of minerals. I do not, however, now 
think on further consideration, that this term, although 
convenient, includes all the cases. In small natural fami- 
lies it is not always easy to recognise any general habit 
or facies at all, and in the case of plants belonging to 
different families where this is the case, but having a 
similar habit, it would be purely arbitrary to fix the 
| pseudomorphism on any of them. Again a// the indi- 
viduals of distinct groups of plants might have a similar 
habit, and the same remark would apply. The difficulty 
is, however, got over by speaking of the plants in these 
cases as zsomorphic. 
My friend, Mr. E. R. Lankester, has pointed out to me 
that agreements of this kind may all come under what he 
| has termed homoplasy (Ann, and Mag. of Natural History, 
July 1870). This is the explanation he gives of this 
expression :— 
“When identical or nearly similar forces, or environ- 
ments, act on two or more parts of an organism which are 
exactly or nearly alike, the resulting modifications of the 
various parts will be exactly or nearly alike. Further, if, 
instead of similar parts in the same organism, we suppose 
the same forces to act on parts in two organisms, which 
parts are exactly or nearly alike and sometimes homo- 
genetic, the resulting correspondences called forth in the 
several parts in the two organisms will be nearly or exactly 
alike. I propose to call this kind of agreement /omo 
plasis or homoplasy. The fore legs have a homoplastic 
agreement with the hind legs, the four extremities being, 
in their simplest form (¢.g. Pvotevs, which must have had 
ancestors with quite rudimentary hind legs), very closely 
similar in structure and function. . . . Homoplasy in- 
cludes all cases of close resemblance of form not traceable 
to homogeny.” 
The resemblances, therefore, above described between 
the vegetative organ of plants with no close generic 
relations, may be described as homoplastic. The difficulty 
* Perhaps one of the most striking is the Natal cycad Stangeria paradoxa 
having been published and described by Kunze asa species of Lomaria, a 
genus of Ferns. 
