Page Ten 



EVOLUTION 



Jur 



1937 



A Lesson In Variation 



By RALPH C. BENEDICT 



Professor of Botany, Brooklyn College 



THIS article outlines a possible laboratory lesson through 

 which the most fundamental factor of evolution 

 may be presented objectively. It has been used a number 

 of years in fourth year high school biology. The facts 

 of morphological resemblances among related forms, geo- 

 graphic distribution, geologic succession of types, embryo- 

 logical and ontogenetic development, plant breeding, etc., 

 are valuable and important as circumstantial evidence, 

 hut an understanding of the basic problem of evolution 

 must be sought in a study of variation as a process. 



If occasional 1\' in reproduction parents produce off- 

 spring which differ from the parent type, and not merely 

 by the re-shuffling of characteristics already possessed by 

 collateral forms, we are brought face to face with the 

 elemental fact upon which any real understanding of evo- 

 lution must be based. 



Variation is one of the numerous words which have 

 a number of different meanings. The word is not used 

 here in the common interpretation as referring to the 

 range of differences between the individuals of a larger 

 species population. The meaning can be sharply delimit- 

 ed to the desired application by the question: Will evolu- 

 tion take place if offspring always repeat the exact char- 

 acteristics of their parents? Why must variation occur 

 as a process in reproduction if new forms are to evolve? 



Is there any evidence of such variation? The Boston 

 fern series furnish excellent material for class study. They 

 are relatively' common, and easy to obtain. The range 



of variation between the different varieties is wide, the 

 differences well marked, and the material is large enough 

 so that the difference can be seen easily. The method of 

 reproduction is entirely vegetative, thus eliminating the 

 possibility of complication through hybridization. 



The mode b\ which variation must have taken place 

 in these fern types can be pointed out easily, and is illus- 

 trated in figure 1. This shows a parent plant of the wild 

 sword fern from which the Boston fern was derived, in 

 association with three offspring which have arisen along 

 a lateral stolon. Such stolons are common in Boston fern 

 varieties, and the methods of vegetative propagation along 

 stolons can usually be demonstrated by digging up a little 

 surface dirt around a well-established pot plant. 



While in general the offspring are practically identi- 

 cal with their parent, a number of times, in the florists' 



Courtesy Brooklyn Botanic Garden. 

 FIGURE 1. Vegetable reproduction of Wild Sword Fern. 

 The three bud plants are all like the parent. In variation 

 one or more would be different. 



FIGIIRK 2. Leaf of Boston Fern (left) with leaves of 

 the seven primary sports. In each case, the original mutation 

 took place in vegetative reproduction (see Fig. 1) 



cultivation of millions of Boston fern plants, an occasional 

 bud plant has arisen which, while still in physical connec- 

 tion with its parent plant, has shown distinct differences 

 from the parent. Figure 2 shows the leaves of a typical 

 Boston fern together with seven such departures or varia- 

 tions. Beginning with the first plant, each of these varia- 

 tions reproduced only its own type, maintaining the differ- 

 ence from the parent Boston fern, and thus representing 

 that kind of variation which is inherited, or mutation. 



This does not establish what the process of variation 

 is; it merely makes obvious the fact of its occurrence, and 

 it is evident also, that whatever happened must have taken 

 place somewhere along the stolon or reproductive branch 

 from which the different buds arose. 



The third figure, showing representative pinnae of the 

 same leaves shown in figure 2, makes clearer just what 

 types of differences have resulted from the variation pro- 

 cess in the Boston fern. These differences parallel to some 

 extent the characteristics vk'hich distinguish recognized fern 



