Briefe. 
121 
Dasselbe gilt von der Blattform; sie ist oft bezeichnend für ganze Gruppen 
von Familien, in der Regel für Familien oder wenigstens für Gattungen, aber 
bisweilen finden sich in derselben Gattung Arten mit der verschiedensten Blatt- 
form, soWeinmannien mit bandförmig ßzähligen, oder 5zähligen, mit gefiederten 
und mit einfachen Blättern, und bei Rhopala hat sogar derselbe Baum bisweilen 
Aeste mit einfachen, andere mit gefiederten Blättern ! — 
Wenn so Charactere, die in der Regel mindestens den Werth von Famiii en- 
characteren haben, oft selbst nicht einmal specifischen Werth haben, so kommt 
auch das Umgekehrte vor. Die Behaarung z. B., die nach Ihres Herrn Vaters 
Feststellungen sicher höchstens specifisch Werth haben könnte und auch in der 
Regel hat, kann zu einem trefflichen Familiencharacter werden, z. B. bei den 
Malpighiaceen, oder gen er ischen Werth bekommen, z.B. bei Indigofera 
An Darwin 1). 
Desterro, April i. 1867. 
My dear Sir 
The last French Steamer brought me your two letters of Febr. 7 and 22, for 
which I am much obliged to you. As to your question about sexual differences 
of lower animals I can hardly give you any Information. Among higher Crusta- 
ceans I know of only one instance of males distinguished by bright colours from 
the females. In a little Gelasimus of our coast the posterior half of the cephalo- 
thorax is often in the adult males of a pure white. This white colour may, in a 
few minutes, change into dirty grey or even black, while at the same time the 
colours of the anterior half lose much of their brilliancy. — The cephalothorax 
of the female is commonly of a nearly uniform greyish brown. 
This little Gelasimus uses to run about in the sunshine and so is raore able 
to exhibit its beauty to the females than are most other Crustaceans. 
The number of the males seems to be much larger, than that of the females; 
just now, when I went to catch some specimens, I brought home a dozen males 
and, only three females. I suspect, that the large pincers serve to this and perhaps 
some other Crabs for fighting for the female; at least, when several males are 
emprisoned in the same glass, then often kill or mutilate one another. 
Among lower Crustaceans the genus Sapphirina Thomps. (S. fulgens, auro- 
nitens etc.), offers a most interesting case in point. The males are known since 
the time of Cook for the brilliancy of their colours, while the females according 
to Gegenbaur and Claus are colourless. In these little Copepods also the colours 
are changing. (See the enclosed extract from Claus.) 
From Gerstäcker's and Carus' Textbook of Zool. I see that in some spiders 
(Sparassus smaragdulus Latr.) the male is distinguished by a different and 
brighter colour. 
Among Annelids some SyUidae with alternate generations show very marked 
sexual differences, so great, that the asexual form, the males and females were 
placed in three different genera (Autolytus Gr. Diploceraea Gr., Sacconereis J. Müll.) ; 
some of the sexual forms have larger eyes and prettier colours, than is usual 
with Annelids and perhaps these colours may be due to sexual selection. — 
I) Nach einem im Nachlass vorgefundenen Concepte. Herausgeber. 
