INTERNAL CAUSES OF VARIATION 



'59 



to many plants and certain animals results in budding and mul- 

 tiplication of parts. We cut the main stem of a small tree or 

 shrub in order to increase the number of side branches. Some- 

 what similarly, injured parts are often doubled in regeneration. 1 

 In this way lizards may be made to produce an increased number 

 of toes and even double feet, legs, and tail. It is supposed that 

 double feet, sometimes seen even in mammals, may be produced 

 by a " fold of the amnion constricting the middle of the begin- 

 ning of the young leg " 2 in the embryo. This, however, is curi- 

 ous rather than valuable to us, as it tends to explain abnormalities 

 rather than to point a way to practical improvement. 



Irregularities in cell division a cause of variation. 3 The char- 

 acteristic act in cell division seems to be the splitting of the 

 chromosomes (or chromatin granules) and the migration of exact 

 equivalents to each new daughter cell, strongly suggesting that 

 the assortment of " physiological units " (whatever they may be) 

 received by one daughter cell is an exact duplicate of that received 

 by the other, thus insuring an orderly and systematic develop- 

 ment through a strictly qualitative division of hereditary sub- 

 stance at each and every stage of growth. 



The whole mechanism of mitosis seems adjusted to this end, 

 and if the assumption is true its significance can hardly be over- 

 estimated. If this careful adjustment of the mechanism of cell 

 division is necessary to orderly development, it is manifest that 

 any substantial deviation is likely, if not certain, to result in 

 variation more or less profound. Such deviation is characteristic 

 of amitotic division generally, and it is more than conceivable 

 that the ordinary process is subject to occasional " slips." Some 

 chromatin granule may fail to divide at the proper moment and 

 may pass over to one daughter cell entire, 4 or, conversely, it 

 may indulge in an extra division. Substantial deviations in the 

 process are known to occur not rarely but frequently. For ex- 

 ample, the splitting- sometimes takes place in the spireme stage, 

 sometimes after the formation of the chromosomes ; sometimes 



1 Morgan, Regeneration, pp. 137-139. 



2 Ibid. p. 139. 



3 See previous chapter. 



4 This is known to occur in certain instances in maturation. 



