EFFECTS OF AMPHIMIXIS ON ONTOGENY 255 



the latter measuring 74 cm.. — the right arm of No. 507 is only 

 71 cm. long, while that of No. 508 reaches 74 cm. The relative 

 lengths of the upper arm and fore-arm are also different, that 

 of the left upper arm of No. 507 being 27 cm., while in No. 508 

 it is 27.5 cm. ; and consequently the length of the fore-arm in No. 

 507 is also 27 cm., while in No. 508 it is only 26 cm. Even if 

 we possessed the measurements of the parents at the same age, 

 we should probably be unable to draw any definite conclusions 

 as to whether these slight differences in size are due to a corre- 

 sponding difference in the combination of the germ-plasm, such 

 as might arise from a slightly inexact division of the nucleus of 

 the fertilised ovum in the process of doubling or at a later stage, 

 or whether they simply owe their origin to slight general or 

 local differences of nutrition taking effect during ontogeny. 



Other facts are, however, known, which prove that although 

 the nature of the combination of the parental idioplasms during 

 ontogeny is in general, as a matter of fact, determined at the 

 time of fertilisation, it is nevertheless liable to slight individual 

 fluctuations. Instances of this kind are furnished by the hybrids 

 of certain species of plants, many parts of which exhibit a con- 

 siderable degree of variability, and fluctuate between the specific 

 characters of the two parents. The blossoms of the hybrid 

 plants obtained by crossing Digitalis liitea and D. purpurea, for 

 instance, ' vary in colour ; in some instances they are pale, with 

 a slight pink tinge, which latter, again, may be entirely absent ; 

 and in others they have a more or less bright purple colour.' * 

 These observations appear to me to be particularly important, 

 owing to the fact that we may assume with certainty in this case, 

 in which two distinct and sharply defined species were crossed, 

 that both parents possessed the specific characters in the same 

 degree of purity and strength, and that consequently the relative 

 proportion of the parental idioplasms does not remain quite con- 

 stant during ontogeny, owing either to slight irregularities in the 

 nuclear division, or — and this is less probable — to inequalities 

 in nutrition and in the growth of the idants derived from the 

 two parents. Owing to the kindness of Professor Hildebrandt 

 of Freiburg i. Br., I have had an opportunity, in the case of 

 hybrids of two species of Oxalis, of observing how extremely 

 detailed the process of predetermination is. The flowers of 



* Focke, ' Die Pflanzen-Mischlinge,' Berlin, 1881, p. 316. 



