156 
NATURE 
[Dec. 22, 1870 


the nearest natural allies of both the mimickers and mimicked 
are ot always to be found in the same district. This 
deserves the more attention, since it appeared so strong to Mr. 
Bates as to lead him to relinquish the idea of hybridisation as an 
explanation after it had crossed his mind. _‘‘ The explanation,” 
says he, “that the whole are the result of hybridisation from a 
few originally distinct species cannot at all apply in this case, 
because the distinct forms, whose intercrossing would be required 
to produce the hybrids, are confined to districts situated many 
hundred miles apart.” 
Before I proceed to show how simple the explanation of the 
absence of one of the parents is, I must beg to note in passing 
the admission that there are distinct forms whose intercrossing 
would produce the hybrids. That granted, I would remind the 
reader of what Mr. Bates has obviously overlooked, that we are 
dealing with a phenomenon probably of a very ancient date, and 
that one side of the parental stock may have disappeared in the 
course of time. I have elsewhere suggested, in regard to hybri- 
disation as a possible originator of species, that it must be a neces- 
sary accession to such an event that the hybrids should have 
opportunity of isolation, such as might be obtained by thinly 
peopled districts where they might settle, spread, and establish 
themselves. Now, certainly, the Valley of the Amazons, the 
Malayan Archipelago, and many parts of the South of Africa 
(lands whence these mimetic analogies come) have at different 
periods all been at one time unoccupied land; for all of 
them have been raised from the bottom of the sea, and been 
peopled by the influx of the inhabitants of neighbouring 
lands. No one knows better than Mr. Bates that at one time 
Brazil was unconnected with New Granada or the Andes. 
The Danaids were then inhabitants of it, but not inhabitants 
of the countries about it ; while the Pieridz, or cabbage whites, 
were what I have elsewhere denominated a microtypal tribe from 
more temperate climes, and were present in the Andes and the 
mountain countries, as Columbia, connected with them. In the 
natural course of things, therefore, when the Valley of the Amazons 
was changed from the bottom of a sea to dry land, the Danaids 
would spread into it from Brazil, and the Pieridz from the north 
and west, and meeting in an open, as yet, unpeopled country, 
hybridisation might take place under one of the few circum- 
stances where I have thought it possible that it could retain its 
place and establish its products as species. The objection that 
irightened off Mr. Bates is, in reality, no objection at all to the 
hypothesis of the mimicry being due to hybridisation, that we 
are not always, or even that we should not at all be able to 
identify the probable parents of the mimickers as inhabitants of 
the same country as their supposed descendants. One of the 
parents we know to be present (the so-called mimicked), but there 
are excellent reasons why the other parent should not be present. 
It is of a northern type, suited for our temperate regions, but 
not adapted to the tropics except at a higher elevation and a 
cooler temperature than the damp, hot valley of the Amazons. 
Although, therefore, it might descend into that region, it is not 
only a natural but almost a necessary inference that it would not 
find it congenial or habitable, and although it might live long 
enough in it to found a dynasty of mimickers, it would soon die 
off from unsuitable conditions, while its hybrid offspring bred 
from the tropical Danaids might, from the black blood so im- 
parted to them, find it sufficiently well suited for them. 
There is yet another phenomenon connected with Mimicry, 
which possibly may also be connected with hybridisation, viz., 
the occurrence of what Mr. Wallace has called dimorphism in 
insects among the mimicking or mimicked species. We must 
not, however, confound this dimorphism with Darwin’s dimor- 
phism in plants. The two are totally different things, and, as it 
seems to me, have no relation or analogy to each other. In 
plants the dimorphism is always confined to the reproductive 
organs, in insects it has apparently nothing to do with them. 
Moreover, it seems to me that all the instances of so-called 
dimorphism in insects that have yet been recorded, are nothing 
but examples of variation, perhaps complicated by hybridisation. 
M. Reinhard, of Bautzen, has shown that this is the case with 
regard to Mr. Walsh’s conclusions respecting the dimorphism 
of certain gall-flies, for he had found that the galls of various 
species appear to be so transitional between other forms, that 
they can only be known with certainty when the perfect insect 
appears. It appears to me to be also the case in all those 
instances where the dimorphism is confined to particular districts, 
as in the Papilio Turnus of North America, where all the females 
are yellow in the New England States and in New York, while 

in Illinois, and farther south, they are all black, and in the 
intermediate region, both black ard yellow females occur in 
varying proportions. And the case is not open to any doubt, 
because in the intermediate district, both yellow and black in- 
sects have been bred from the same batch of eggs. Now, if the 
case had been that doth males and females equally varied, 
and that in the south all were black and in the north 
all yellow, with intermediate gradations in the districts between, 
we scarcely suppose that any one would have thought of calling 
it a case of dimorphism. If they did, then all climaial varia- 
tions (and their name is legion) would come under the same 
category. It is only dimorphism, because the change is limited 
to the female. But is this a good ground? Physiologists are 
unanimous in holding that neither the male nor the female is the 
species, but both ; and if that be the case, in what does a varia- 
tion in the female and not in the male differ from a yariation in 
both but in degree? Most of Mr. Wallace’s instances of dimor- 
phism are of this character—the male being the same in a 
number of islands in each of which the female differs. All 
these I regard as mere instances of climatal variation, in which 
the variation shows itself only in that part of the species called 
the female. An occasional case of variation from some other 
cause, as from hybridism, may possibly come to complicate this 
phenomenon ; but it appears to me to be sufficiently explained 
by variation, and the circumstance above mentioned is significant 
that where mimicry occurs in species having dissimilar sexes, it 
too is often confined to the female. A. Murray 


SCIENTIFIC SERIALS 
Silliman’s Journal, September 1870.—The opening article 
of this number is by Prof. E. Loomis, and is entitled ‘* Compa- 
rison of the mean daily range of the Magnetic Declination, with 
the number of Auroras observed each year, and the extent of 
the black spots on the surface of the Sun.” The author first 
discusses the observations of sun-spots, and points out some cor- 
rections that should be made in the numbers obtained by astro- 
nomers in the last century ; he points out that the period is one 
of ten years, and is influenced by the heliocentric conjunctions 
of Jupiter and Saturn, but affected by the conjunctions of the 
Karth and Venus. By a series of tables and curves the coinci- 
dences of periods of the maximum number of sun-spots with the 
maxima of magnetic disturbance and auroral display are eluci- 
dated, from which it appears that the present year is a period of 
maximum.—In a letter to the editors, Mr. J. W. French 
proposes a new period in chronology called the Precession Period, 
of 25,782 years, being the time for the precession of the 
equinoxes, The author prefers this period, since it is founded 
solely on astronomical facts.—-The third article is by F. W. 
Clarke, ‘“ On the atomic volumes of solid compounds,” in which 
are discussed the relations of the volumes of analogous and simi- 
larly constituted bodies. —Thenext article, ‘‘Considerations onthe 
apparent inequalities of long periods in the mean motion of the 
Moon,” is by Simon Newcomb, and, after a long discussion on 
the observations on this subject, and the theories proposed to 
explain them, the author attributes the phenomenon to an irre- 
gularity in the rotation of the crust of the earth, caused by the 
motion of its fluid contents. —The following is a very interesting 
article by Dr, A. M. Mayer on ‘Researches in Electro- 
magnetism.” The author has devised a very accurate method of 
determining the relative values of electro-magnets to replace the 
one usually employed, which consists of measuring the deflection 
of a magnetic needle which is produced by the action of the 
electro-magnet. The author found that this process was liable to 
error in consequence of the difficulty of keeping the current 
absolutely constant, resulting in a continual motion of the needle. 
These difficulties were obviated in the following manner: A 
line eight feet long and divided into fractions of inches was 
drawn on a table, the latter being so placed that the line was at 
right angles to the magnetic meridian ; a compass, with a needle 
nearly six inches long, was placed on this line, and a helix was 
fixed at each extremity of the line. These helices were traversed 
by the same current, a tangent galvanometer being placed in 
the circuit. In this way the needle was influenced by two 
magnets acting in opposite directions and excited by the same 
current, and if any deflection of the needle was observed, it 
must have been due to a difference of power of the magnets. 
If this occurred the needle might be brought to 0° by moving 
it from the stronger magnet. A series of experiments was made 

