! 7 6 CAUSES OF VARIATION 



essential differences in children of the same parents, even in 

 the case of twins. 1 



It is true, of course, that no two individuals, even twins, can 

 be developed under conditions of life exactly identical ; and yet 

 the differences of condition cannot account for the fact that, 

 while one brother resembles his father, another may resemble 



1 Twins are considered as arising from separate ova, as in the case of multiple 

 births (pigs, dogs, etc.), and, of course, as exhibiting the deviations to be expected 

 from different germs and distinct fertilization, as in litters generally. 



Some twins, however, are so nearly alike (identical twins) as to suggest the 

 possibility of their having arisen from a single ovum in some way separated into 

 halves at its first cleavage, each half developing an individual. This view is 

 evidently favored by Geddes and Thomson (see The Evolution of Sex, p. 41). 



If twins should be developed in this manner, they would evidently be of the 

 nearest possible similarity, for they represent but one ovum and but a single 

 fertilization. 



This possibility is supported by the experiments of Roux, Endres, and Walter, 

 in which each blastomere of the two-cell stage of the frog sometimes (not always) 

 is capable of developing into a perfect individual. Driesch, working with echino- 

 derms, established the same facts, which are also well known in the case of 

 Amphioxtts (see Wilson, The Cell, p. 419). 



Conversely, when two fertilized ova of sea urchin, or Ascaris, adhere acci- 

 dentally, they may develop into an embryo of unusual dimensions (see Loeb, 

 Studies in General Physiology, Part II, p. 676). 



When, however, the blastomeres are not separated, but one of them is killed 

 by a heated needle (Roux), the uninjured half alone develops, but it produces 

 at the best a kind of half larva (right or left half), " containing one medullary 

 fold, one auditory pit " (Wilson, The Cell, p. 399). Chun, Driesch, Morgan, and 

 Fischel, working with ctenophore eggs, however,found that isolated blastomeres 

 of the two-, four-, or eight-cell stages developed " defective larvae, having only 

 four, two, or one row of swimming plates." Also " Crampton found that in the 

 case of the marine gasteropod llyanassa isolated blastomeres of two-cell or four- 

 cell stages segmented exactly as if forming part of an entire embryo, and gave rise 

 to fragments of a larva, not to complete dwarfs as in the echinoderm" (Wilson, 

 The Cell, p. 419). This attempt to form entire individuals from a portion only 

 of a fertilized egg, resulting as it often does in dwarfs, seems to the writer a 

 process closely akin to regeneration (which see in chapter on " Relative Stability 

 and Instability of Living Matter"), and would seem to raise doubts as to its 

 successful occurrence in the higher animals. 



Though not bearing especially upon the point in question, the matter of twins 

 in cattle is unique and worthy of mention. Three kinds of twins are known in 

 cattle: " (i) the twins may be both female and both normal; 0^(2) the sexes 

 may be different and norm'al ; or (3) both may be males, in which case one always 

 exhibits the peculiar abnormality known as a 'free-martin,' the internal organs 

 are male, but the external accessory organs are female, and there are also rudi- 

 mentary female ducts." (Geddes and Thomson, The Evolution of Sex, p. 41). 

 This is a kind of hermaphroditism, and not, as is commonly supposed, "a heifer 

 twin with a bull." 



