Page:Catholic Encyclopedia, volume 5.djvu/740

 EVOLUTION

662

EVOLUTION

than individual differences; for they are discontinu- ous and constant, and are therefore capable of explain- ing the gaps between existing species and those of palipontologj'. We use the term single variation when, from among a large number of offspring, some one particular individual stands out that differs from the rest in one or more characteristics which it trans- mits unchanged to posterity. It is said to be peculiar to the single variations that they cannot be reduced to crosses. If this is possible, we speak of " analytical variations". Favourable conditions for the appear- ance of single variations are altered environment, a liberal sowing of seed, and excellent nourishment. It is a remarkable fact that the fertility of single varia- tions decreases considerably, and this the more so the greater the deviation from the parents. Besides, the newly produced forms are comparatively weak. This weakness and inclination to sterility are facts which must be carefully weighed when determining the probable importance of single variations for specific evolution. Besides, it is — to our knowledge — in no case excluded that the suddenly arising form may be traced back to former crossings. Probably the only case which is quite generally interpreted to demon- strate specific evolution experimentally is that of the prunrose observed by de Vries. After many failures with more than 100 species, de Vr;es, in ISSG, deter- mined to cultivate the evening primrose (CEnothera Lamarckiana), whose extraordinary fertility had at- tracted his attention. He chose nine well-developed specimens and transplanted them into the Botanical Garden of Amsterdam. The cultivation was at first continued through eight generations. In all he exam- ined .50,000 plants, among which he discovered SOO deviating specimens, which could be arranged in seven different groups, as shown in the following table:—

and darker yellow, or smaller and lighter, the fruit longer or shorter, the outer skin rougher or smoother, etc.

It may be conceded that the Oenothera has devel- oped constant forms corresponding to the so-called "small or elementary species". The question, how- ever, is, whether the forms are really new ones or whether they owe their origin to some unexpected original cross. In fact, if we are to suppose a previous cross, perhaps 0. Lamarckiana and 0. sublinearis, then the 0. Lamarckiana of Hilversum had contained the different variations in a latent form and through culti- vation gradually reverted by throwing off the different variations. At any rate, there cannot be any ques- tion of a progressive development, for the reason that none of the new forms shows the slightest progress in organization or even development of any kind advanc- ing in that direction.

(3) Crosses and Mendelian Segregations. Cross- breeding can in nature hardly be considered as a factor in the progressive development of species ; in particu- lar, forms of different degrees of organization do not cross, and if they did, all deviations would soon be equalized according to the laws of chance and proba- bility. All the greater seems to be the importance of the jlendelian segregations. It may be kno\\Ti to the reader that the famous experiments of the Abbot Mendel were carried on with seven different pairs of characters which he crossed with one another, and then, by letting the cross-breds self-fertilize, he contin- ued the cultivation of the plants through a series of generations. In the first generation it was found that the offspring exhibited without exception the charac- ter of one of the parents, that of the other parent not appearing at all. Mendel therefore called the former — the prevailing — character the "dominant" and the other the "recessive". In the following generation,

Generation

O. gigas

albida

oblonga

rubrinervis

Lamarckiana

nanella

lata

scintillans

I. 1S86-S7



-



-

9

—

—

-

II. 1888-89

15000

5

5

—

III. 1890-91

1

10000

3

3

—

IV. 189.5

1

15

176

8

■ 14000

60

73

1

V. 1S96

2.5

135

20

8000

49

142

6

VI. 1S97

11

29

3

1800

9

5

1

Vir. 1898

9

—

3000

li

—

VIII. 1899

5

1

1700

1

-

ThespecimenofO.ji'jas (1895) was self-fertilized and yielded 4.50 O. gigas forms, among which there was only one dwarf form, 0. gigas-nanella. The three fol- lowing generations remained constant. O. albida was a very sickly form, though it succeeded, thanks to reg- ular attention, in breeding constant offspring. Among the 0. oblonga descendants there w-as one specimen, albida, and in a later generation one specimen of O. rubrincrris. 0. rutirincrvis proved to be as fertile as Lamarckiana, and yielded besides a new variation, leptocnrpa. The offspring of O. nanella was constant, though among the 1800 ilescendants of nanella in 189G three specimens showed oblonga characteristics. 0. lata was purely female; but, fertilized with pollen of other variants, it yielded 15 to 20 per cent 0. lata descendants. O. scintillans was not constant. Ac- cording to de Vries' observations fsince 188C), new forms also originatcil in nature, but they succumbed in the struggle for existence. The differences between the single forms relate to various parts and degrees of development, though in .several they are very slight. The plants become either stronger or weaker, with broader or narrower leaves; the flowers become larger

which was produced by letting the cross-breils fertilize themselves, the recessive character appeared and, moreover, in a definite proportion. On an average this proportion was 2.89 : 1 or 3 : 1. In the second generation 75 per cent of the whole number of plants exhibited the dominant character, and 25 per cent the recessive. No interniei.liate form.s were observed in any case. In the third generation the off.spring of the recessives was constant and remained pure recessives, but among the offspring of the dominants some remained constant dominants, while others were hy- brids. The average proportion of the constant domi- nants (D) to variable cross-breds (DR) was as 1 : 2. Thus, besides the 25 per cent of constant recessives (R), there was also 25 per cent (one-third of 75 per cent) constant dominants (D) and 50 per cent (two- thirds of 75 per cent) variable crossbrcds (DR) or 1D+ 2DR4- IR. The same proportion resulted from the following generations of the crossbreils, and since 1900 this has been confirmed by otlier investigators in the case of other plants (e. g. maize) and al.so of ani- mals (e. g. gray and white mice). Mendel's rule of segregation, therefore, runs thus: