Page:Catholic Encyclopedia, volume 5.djvu/737

 EVOLUTION

659

EVOLUTION

Hugo de V

'definitely' inherited variations suddenly appear, some of which will find an environment to which they are more or less well fitted, we can see how evolution may have gone on without assuming new species to have been formed through a process of competition. Nature's supreme test is survival. She makes new forms to bring them to this test through mutation and does not remodel old forms through a process of indi- vidual selection." We shall see that de Vries overrated the importance of his experiments. Still it is not to be denied that he has become through his method a mas- ter for the experi- mental investiga- tion of the prob- lems of evolution. Of especial value is his analysis of the concept of species, though probably his greatest ser- vice is the redis- covery of Mendel 's laws and their in- troduction into the realm of bio- logical investi- gations.

The earliest forerunners of Mendel were the first scientific hybridists J. G. Kohlreutev (1733-1806) and T. A. Knight (1758-1838). Kohlreuter's results are of special interest because, through the repeated cross- ing of a hybrid with the pollen or ovules of one of the parents, forms appeared which more and more re- verted to the characteristics of the respective parent. K. F. von Gartner (1772-1850) was the most prolific writer on hybridism of his time, though he did not sur- pass Kohlreuter as to the positive results of his ex- perimental research. C. Naudin's essay on the hy- bridity in plants (1862) represented a considerable ad- vance. The author pointed out that the facts of the reversion of the hybrids to the specific forms of their parents, when repeatedly crossed with the latter, are naturally explained by the hypothesis of the segrega- tion of the two specific essences in the pollen grams and ovules of the hybrids (Leek). This formed in after years no small part of Mendel's discovery, which is indeed one of the most brilliant results of experi- mental investigation.

Gregor Mendel was born 22 July, 1822, at Heinzen- dorf near Odrau (Aastrian Silesia). After finishing his studies he entered, in 1843, the Augustinian mon- astery at Briinn. Having been for fourteen years professor of the natural sciences, he was elected abbot of the monastery in 1868, and died in January, 1884. Mendel's celebrated memoir, " Versuche iiber Pflan- zenhybriden", appeared in 1865, but attracted little attention, and remained unknown and forgotten till 1900. It was ba.sed on experiments that had been carried out during the course of eight years on more than 10,000 plants. The principal result of these ex- periments was the recognition that the peculiarities of organisms produced entities independent of one an- other, so that they can be joined and separated in a regular way. As we have said above, H. de Vries was the first to recognize the value of Mendel's paper. Other investigators who have taken up the same line of work are Correns, Tschermak, Morgan, and, most of all, Bateson, the principal founder of "Mendelism", or the science of genetics.

II. Definition of Species. — Before Linnaeus's time genera were considered to be the units of the plant and animal kingdoms, and it was assumed these had been created by tiod, while the species were descended from them. By the nomen speci/icum was understood the more or less short description by which Tournefort and his contemporaries distinguished the various species of genera. Linnseus introduced the binomial system establishing the species as the unit of the organic world. There are as many species as there were different forms created in the beginning. The same theoretical norm had already been adopted before Linnaeus by the Eng- lish physicianJohnRay(died 1G78). The practical crite- rion for determining genera and species was taken from characteristic morphological features. For instance, the essential generic characteristic of the quadrupeds was derived from the teeth ; that of birds from the bill. The species was designated in a similar manner " by retaining the primary characteristic among the vari- ous differences which separated two individuals of the same species." The establishment therefore of a genus or of a species depended ultimately, then as now, on the knowledge and subjective views of the systema- tizer. The whole system was an artificial one pre- cisely because it took note of one single feature alone, leaving the rest out of consideration; for instance, in the vegetable kingdom the character of the flower alone was taken into consideration. Later on Lin- nseus entertained the idea that originally God created only one species of each genus, and that the rest had been derived from these original species by cross- breeding. Linnseus's conception of species was strengthened by Georges Cuvier, who defended the unchangeableness of the categories beginning with the species up to the four types (embranchement). He was supported in this, as was later L. Agassiz, by the absolute dearth of intermediate forms in geological strata. Hence arose his Theory of Catastrophes, which in turn gave way to his Migration Theory. Cuvier came victorious out of the controversy with Etienne Geoflroy Saint-Hilaire, who maintained the unity of the plan of animal structure and the continu- ous transition of forms in the ani- mal kingdom.

The views pre- vailing under Lin- naeus and Cuvier were then divided into two main branches. (l)The more moderate Transmutationists held that genera were the originally created units, and that from these all species and vari- eties were derived. (2) The followers of Linneeus, on the other hand, af- firmed that the Linna;an species were the created units, and the sul> divisions of these were the derived ones. Then fol- lowed the Jordan schools, which asserted that within the Linnaean species were what they called "small species", in- dividtially variable, but specifically immutable (not connected by intermediate forms), and, as such, to be considered the true units or " elementary species ". Linnaeus's Draba verna, for instance, comprehends about 200 "elementary species". The norm or

Gregor Mendel