VARIATION. ADAPTATION. ORIGIN OF SPECIES 



393 



again reduced, and it is only after long-continued exposure to high temperatures 

 that a constantly asporogenous race is formed. 



Let us now turn to the third type of variation, viz. mutations. These have 

 come into special prominence of late years owing to the labours of De Vries 

 (1901) and KoRSCHiNSKY (1901). By mutations we understand variations 

 which, once initiated, are thereafter quite constant. Mutations are variations 

 which appear spasmodically, one or more characters of a species suddenly 

 exhibit alteration or appear ah initio, and these may show themselves for the 

 first time in a plant which has arisen from a seed or a single bud. Let 

 us consider first of all a few examples of malformations which appear to be of 

 the nature of mutations. Look at the plants which possess branched leaves. 

 The oldest example is Chelidonium laciniatum, a form of Chelidonium majtis 

 which suddenly made its appearance at Heidelberg in 1590, a form possessed 

 of branched leaves, both floral and vegetative, and which has constantly 

 reproduced these characters by seed from that day to this. The double flowers 

 which occur in many plants have arisen by mutations, as also cases of 

 fasciation, best known in the commonly cultivated plant Celosia cristata. 

 Further, very characteristic examples are seen in the rayless Compositae 

 e.g. Matricaria discoidea, the thornless varieties of plants which usually 

 possess thorns, e.g. on the fruit {Ranunculus arvensis inermis, Datura tatula, &c.). 

 Again, we have the notable case of Capsella heegeri, which was found growing 

 wild near Landau, and which has been shown (Solms-Laubach, 1900) to be 

 a mutation of Capsella bursa-pastoris. As an example of bud variation equally 

 well known we may select the case of Sedum reflexum, recorded by Wettstein 

 (1900) as having been found near Prag, which showed fasciation on a single 

 lateral branch. This branch, when cultivated, flowered, and from the seed 

 arose again magnificently fasciated examples. 



The reason why mutations are of so much importance for the theory of the 

 origin of species is especially because, in addition to the constancy of the new 

 character, the innovation is in no sense an adaptation but entirely a mark of 

 organization. The mutations cited, however, differ certainly in one charac- 

 teristic only from the parent species, such as we find in those natural forms 

 termed by many authors varieties (e.g. white-flowered varieties), but not in ' ele- 

 mentary ' or sub-species, which usually exhibit differences in all characters. The 

 direct observation of a new sub-species by mutation is to be considered as a great 

 advance in our knowledge, and this observation we owe to De Vries (1901 a). 

 He cultivated many specimens of Oenothera lamarckiana, an American immi- 

 grant into Europe, and now partly naturalized there, and was able to establish 

 the occurrence of many mutations which differed from the type in many or in all 

 characters. The offspring arising from one seed were constant in exhibiting the 

 new character or characters. We shall limit ourselves here to the consideration 

 of one example only, viz. Oenothera gigas. Let us hear what De Vries (1901 b) 

 has to say on the subject : — ' It is of the same height as the parent species, but the 

 stem is thicker, the leaves are more numerous, the corolla is more widely opened, 

 and the buds are much thicker (compare Figs. 116 and 117), the fruits are only 

 half as long as the fruits of the parent, and they contain fewer seeds. The seeds 

 are larger, rounder, and heavier. This type arose in my garden in the year 

 1895 as a solitary case amongst 14,000.' By preventing any crossing, pure 

 seeds of this type were gathered in 1896. 'These were planted in 1897. As 

 soon as the third and fourth leaves had unfolded, the differences appeared. All the 

 young seedlings were stronger and more ftdly clothed with leaves and darker in colour 

 than the parent. There were several hundred such, but obviously all of one type 

 only, and as in the course of the summer, first the stem and then in succession 

 the leaves, the buds, the flowers, and, finally, the fruits showed themselves, all 

 doubt was removed that a new and constant species had made its appearance. 



