654 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Species and Varieties : their Origin by Mutation.* — Hugo de 

 Vries has given a luminous exposition of his discoveries and conclusions. 

 Some of the salient features are summed up in an American review of 

 the work. 



Linnasan, or collective species, are abstract averages of' a number 

 of types. It is often necessary to segregate off from these a number of 

 elementary species. These are contrasted with varieties in that they 

 differ in more than one respect, and possess qualities which are dis- 

 tinctly new. A variety may be based on a single quality, more 

 frequently negative than positive. Varietal characters represent 

 physiological units, appearing and disappearing singly. In crossing 

 varieties all the characters are paired, and the progeny follow the 

 Mendelian law of splitting. Such crosses may be termed bisexual, to 

 contrast them with the unisexual crosses of elementary species which 

 result in constant hybrids. Sports originating from varieties do not 

 introduce anything really new. The wide range of variability in 

 " eversporting varieties " is due to the presence of mutually excluding 

 characters, by reason of which the forms swing from one extreme to the 

 other. In most cases, however, latency of the more or less absent 

 character not being complete, there are intergrading forms, and thus 

 such sports are not really new. 



In his careful cultivation experiments, notably with (Enothera 

 lamarckiana, de Vries found that markedly distinct forms or mutants 

 may suddenly arise, and may continue to breed true for successive 

 generations. These new elementary species arise abruptly without 

 intermediate steps. New forms spring laterally from the main stem. 

 New elementary species attain their full constancy at once ; they are 

 produced in a large number of individuals ; they may occur in nearly 

 all directions. Some new strains arising by mutation are to be ranked 

 as variations. " The great difference between this and the Darwinian 

 theory of the origin of species is that here we have new forms which 

 are to be recognised as specifically distinct, arising in perfectly constant 

 form, by sudden leaps, or more properly mutations, rather than by con- 

 tinuous slow variations. This, and the fact that many individuals, 

 whole species indeed, are mutating simultaneously, must profoundly 

 modify the Darwinian concept." 



b. Histology. 



Tapetum of Abramis brama.j — S. Exner and H. Januschke have 

 studied the changes in the pigment-epithelium of the retina of this fish 

 under different conditions of illumination. In darkness there is a 

 definite change of shape — the thread- like or club-like processes are 

 shortened, and their content of fuscin and guanin undergoes rearrange- 

 ment. The granules of the former migrate backwards between the 

 granules of the latter, and accumulate in the basal region of each cell. 



* Species and Varieties, their Origin by Mutation. By Hugo de Vries. 

 Edited by D. T. MacDougal. Chicago, 1905, xviii. and 847 pp. See also Review 

 by H. M. R., Amer. Nat., xxxix. (1905) pp. 747-51. 



t S.B. Akad. Wiss Wien, cxiv. (1905) pp. 693-714 (1 pi.). 



