GENETIC CHANGES 207 



more careful study of the case makes it seem probable tliat 

 we are here dealing with a pathological condition of the 

 cytoplasm rather than with true inheritance. Consider a 

 similar case in animals. The organism which produces Texas 

 fever in cattle is introduced into the blood of cattle b\' tlic 

 bite of a diseased tick. Among ticks, the disease passes from 

 mother to offspring in the cytoplasm of the egg. The sperm is 

 too small to carry the disease germ, and so the disease does 

 not pass from father to offspring in the sperm. In reciprocal 

 crosses between diseased and healthy ticks, if such could be 

 made, we should observe exactly the same mode of trans- 

 mission as in the four-o-clock crosses between white branches 

 and green branches. The offspring would always show the 

 condition of the mother, never that of the father. But we 

 should hesitate to describe the transmission of a foreign or- 

 ganism in the egg of a tick as inheritance, and the same 

 hesitancy should be shown regarding the transmission of 

 diseased chloroplastids in the cytoplasm of Mirabilis} 



Leaf -variegation, quite similar in appearance to that just 

 described, but of truly genetic origin, occurs also in Mirabilis 

 and was studied simultaneously by Correns. This is trans- 

 mitted alike in egg-cell and pollen, as a recessive character, 

 which shows that the gene concerned is probably borne in 

 the nucleus. In certain other plants (Antirrhinum, Melan- 

 drium) leaf -variegation is a dominant character transmitted 

 equally by both sexes. In fact, in a great majority of cases, 

 variegation is inlierited as an ordinary Mendelian character, 

 either dominant or recessive, and so may be explained as 

 due to genes contained in the nucleus. The exceptional 

 cases are cases of cell pathology rather than of inheritance. 



If then, as seems probable, genes located in the chromo- 

 somes constitute the sole vehicle of inheritance, it follows 

 that heritable variations can arise only from changes in the 

 genes. Such changes are called *' mutations," of which we 

 can distinguish the following varieties: 



1 Similar cases of "maternal inheritance" have been studied by Baur in Antirrhi- 

 num, by Gregory in Primula, by Ikeno in Plantago, and by Winge in Hunmlus. 



