The Making of Species

by Douglas Dewar; Frank Finn

		H		H
		|		|
H	––	C	––	C	––	O	––	H = C2H6O,
		|		|
		H		H

and methylic ether by the structural formula:—

		H				H
		|				|
H	––	C	––	O	––	C	––	H = C2H6O.
		|				|
		H				H

The formulæ indicate a very different arrangement of the nine atoms which compose the molecule in each case. And to this different arrangement the differing properties of the two compounds are supposed to be due. A rough illustration of the phenomenon of isomerism is furnished by written language. Thus, three different words can be made from the letters t, a, and r, e.g. tar, art, and rat. They also form tra, which does not happen to be an English word, although it might have been one.

Experiments of Gräfin von Linden

Among organisms we sometimes observe a phenomenon which looks very like isomerism. The classical example of this is furnished by the butterflies Vanessa prorsa and Vanessa levana.

At one time these were supposed to belong to different species, since they differ so greatly in appearance. Vanessa levana is red, with black and blue spots. Vanessa prorsa is deep black, with a broad yellowish-white band across both wings. It is now known that the levana is the spring form and the prorsa the summer and autumn form of the same species. The pupæ of levana produce the prorsa form, but Weismann found that after being placed in a refrigerator they emerged, not as prorsa, but partly as levana and partly as another form intermediate in many respects between levana and prorsa. Weismann also succeeded, by exposing the winter pupa to a high temperature, in making it give rise to the prorsa form, and not to the levana form, as it would ordinarily do.

Similar results have been obtained with the seasonally dimorphic Pieris napi. Standfuss, the Gräfin von Linden, and others have obtained like results in the case of other seasonally dimorphic butterflies. In some instances it has been proved that the change in the pigment is a purely chemical one; a similar transformation can be effected in the extracted pigment. But, we must bear in mind that the changes which are induced in this way are not confined to colour; they occur in the marking and shape of the wing.

Even more remarkable is the fact that in some sexually dimorphic species a change of temperature alters the female, so as to cause her to have the outward appearance of the male. For example, it has been found that warmth changes the colours of the female Rhodocera rhamni and Parnassius apollo into the colours of the male.

By applying rays of strong light, electric shock, or centrifuge, the Gräfin von Linden was able to change the colours of the butterflies to which the caterpillars gave rise. Pictet experimented on twenty-one species of butterflies, or rather on their caterpillars, and found that in nearly all cases when the caterpillars ate unusual food, they developed into butterflies with abnormal colouring. Schmankewitsch made the discovery that, in the case of the crustacean Artemia, he could produce either of two species according to the amount of salt in the water in which these creatures were placed. He declared that the anatomical differences between the species Artemia salina and Artemia milhausenii depended solely on the percentage of the salt in the surrounding water. He further stated that by adding still more salt he could change the Artemia into a new genus—Branchipus. More recent observers have cast doubt upon these results of Schmankewitsch. They, however, admit that the degree of salinity of the water has some effect on the form of the Artemia, although they suggest that factors other than concentration affect the result. In any case, it is now well-known that changes in the environment effect changes in the colouring of many crustacea. Pictet has shown that the alternating wet and dry seasons in some tropical countries are the cause of, or stimulus that induces, seasonal dimorphism in some butterflies. He was able to effect changes in the colouring of certain species by means of humidity.