32 
SCIENCE. 
A DARWINIAN STUDY. 
By Alfred R. Wallace. 
For the benefit of those unacquainted with entomology we 
may state, that many butterflies have two, or even three 
broods in a year. One brood appears in spring, their 
larvae having fed during the preceding autumn, and passed 
the winter in the pupa state, while the others appear later 
in the year, having passed rapidly through all their trans- 
formations and thus never having been exposed to the cold 
of winter. In most cases the insects produced under these 
opposite conditions present little or no perceptible differ- 
ence ; but in others there is a constant variation, and some- 
times this is so great that the two forms have been described 
as distinct species. The most remarkable case among 
European butterflies is that of Araschnia prorsa , the winter 
or spring form of which was formerly considered to be a 
distinct species and named Araschnia levana. The two 
insects differ considerably in both sexes, in markings, in 
color, and even in the form of the wings, so that till they 
were bred and found to be alternate broods of the same 
species (about the year 1830) no one doubted their being 
altogether distinct. 
In order to learn something of the origin and nature of 
this curious phenomenon Dr. Weisman has for many years 
carried on a variety of experiments, breeding the species 
in large numbers and subjecting the pupae to artificial 
heat or cold for the purpose of hastening or retarding the 
transformation. The result of these experiments is, that 
by subjecting the summer brood to severe artificial cold in 
the pupa state, it may be made to produce perfect insects 
the great majority of which are of the winter form, but, on 
the other hand, no change of conditions that has yet been 
tried has any effect in changing the winter to the summer 
form. Taking this result in connection with the fact that 
in high latitudes where there is only one brood a year it is 
always the winter form, Dr. Weismann was led to the hypo- 
thesis that this winter form was the original type of the 
species, and that the summer form has been produced 
gradually, since the glacial epoch, by the summer becom- 
ing longer and thus admitting of the production of a second 
or summer brood. This explains why the production of 
the winter form (A. levana) from summer larvae is easy, it 
being a reversion to the ancestral type ; while the produc- 
tion of the summer form {A. prorsa) from autumnal larvae is 
impossible, because that form is the result of gradual de- 
velopment ; and processes of development which have 
taken thousands ofyears to bringabout cannot be artificially 
reproduced in a single season. 
This hypothesis was supported by experiments with 
another two-brooded species, Pieris napi, with similar re- 
sults, the winter form being produced with certainty by the 
application of cold to summer pupae ; and Mr. Edwards, in 
America, has made similar experiments with the various 
forms of Papilii ajax , finding that the summer broods can 
be changed into the winter form by the application of cold, 
while the winter broods can never be made to assume the 
summer form by hastening the process of transformation. 
In the Arctic regions and in the high Alps there is only 
one form of Pieris napi, which very closely resembles the 
winter form of the rest of Europe, and this could never be 
the least changed by rapidly developing the pupae under 
the influence of heat. 
Another curious case is that of one of the Lycaenidae 
{Plebeius agestis) which exhibits three forms, which may be 
designated as A, B, and C. The first two, A and B, are 
alternate broods (winter and summer) in Germany, while in 
Italy the corresponding forms are B and C, so that B is the 
summer form in Germany and the winter form in Italy. 
Here we see climatic varieties in process of formation in a 
very curious way. 
That temperature during the pupa stage is a very power- 
ful agent in modifying the characters of butterflies, is well 
shown by the case of Polyommatus phlaas. The two broods 
of this insect are alike in Germany, while in Italy the sum- 
mer brood has the wings dusky instead of copper-colored. 
The period of development is exactly the same in both 
countries, so that the change must, it is argued, be attribut- 
ed to the higher temperature of the Italian summer. It has 
been noticed that in Italy a large number of species of 
butterflies are thus seasonally dimorphic which are not so 
in Central and Northern Europe. 
Dr. Weismann lays great stress on the varied effects ol 
temperature in modifying allied species or the two sexes 
of the same species, from which he argues that the essential 
cause of all these changes is to be found in peculiarities of 
physical constitution, which cause different species, varieties, 
or sexes to respond differently to the same change of temp- 
erature ; and he thinks that many sexual differences can be 
traced to this cause alone without calling in the aid of sex- 
ual selection. The general result arrived at by the labor- 
ious investigation of these phenomena is, that — ‘a species 
is only caused to change through the influence of changing 
external conditions of life, this change being in a fixed 
direction which entirely depends on the physical nature of 
the varying organism, and is different in different species, 
or even in the two sexes of the same species;” and he adds : 
— “ According to my view, transmutation by purely intern- 
al causes is not to be entertained. If we could absolutely 
suspend the changes of the external conditions of life, 
existing species would remain stationary. The action of 
external inciting causes, in the widest sense of the word, is 
alone able to produce modifications ; and even the never- 
failing ‘ individual variations,’ together with the inherited 
dissimilarity of constitution, appear to me to depend upon 
unlike external influences, the inherited constitution itself 
being dissimilar, because the individuals have been at all 
times exposed to somewhat varying external influences.” 
The present writer has arrived at almost exactly similar con- 
clusions to these, from a study of the geographical distri- 
bution and specific variation of animal forms, as stated in 
an article on “ The Origin of Species and Genera,” which 
appeared in the Nineteenth Century of January last, and it 
is gratifying to find them supported by the results of a very 
different line of inquiry, and by the authority of so eminent 
and original an observer as Dr. Weismann. 
A FOURTH STATE OF MATTER 1 
In introducing the discussion on Mr. Spottiswoode and 
Mr. Moulton’s paper on the “Sensitive State of Vacuum 
Discharges,” at the meeting of the Royal Society on April 
15, Dr. De La Rue, who occupied the chair, good-naturedly 
challenged me to substantiate my statement that there is 
such a thing as a fourth or ultra-gaseous state of matter. 
I had no time then to enter fully into the subject ; nor was 
I prepared, on the spur of the moment, to marshal all the 
facts and reasons which have led me to this conclusion. 
But as I find that many other scientific men besides Dr. De 
La Rue are in doubt as to whether matter has been shown 
to exist in a state beyond that of gas, I will now endeavor to 
substantiate my position. 
I will commence by explaining what seems to me to be 
the constitution of matter in its three states of solid, liquid, 
and gas. 
I. First as to Solids : — These are composed of discon- 
tinuous molecules, separated from each other by a space 
which is relatively large — possibly enormous — in compari- 
son with the diameter of the central nucleus we call molecule. 
These molecules, themselves built up of atoms, are governed 
by certain forces. Two of these forces I will here refer to — 
attraction and motion. Attraction when exerted at sensible 
distances is known as gravitation, but when the distances 
are molecular it is called adhesion and cohesion. Attraction 
appears to be independent of absolute temperature ; it in- 
creases as the distance between the molecules diminishes ; 
and were there no other counteracting force the result would 
be a mass of molecules in actual contact, with no molecular 
movement whatever — a state of things beyond our concep- 
tion — a state, too, wh ; ch would probably result in ihe crea- 
tion of something that, according to our present views 
would not be matter. 
This force of cohesion is counterbalanced by the move- 
ments of the individual molecules themselves, movements 
1 “ On a Fourth State of Matter,” in a letter to the Secretary of the 
Royal Society. By W. Crookes, F.R.S, 
