less i] 



FACTORS OF INHERITANCE. 49 



less intermediate in appearance between the two parent 

 forms, or like neither of them (as in the case of the Blue 

 Andalusian) ; or again they may closely resemble one of 

 the parents. Indeed the resemblance of the heterozy- 

 gotes to one of the homozygote parents may be so com- 

 plete as to deceive a keen observer. For instance, as 

 shown by Mendel himself, if two races of the common 

 pea (Pisum sativum) be crossed, one being tall and the 

 other dwarf, tne first heterozygote generation will all be 

 tall. These interbred yield one-quarter dwarf plants, 

 and three-quarters tall plants. Of these tall plants one 

 in every three breeds true to tallness, is a tall homozy- 

 gote, while the other two prove to be heterozygotes, 

 splitting again into the three kinds. The character tall- 

 ness, then, always shows itself in those individuals to whom 

 its factor has been transmitted, whether they be homo- or 

 heterozygotes. It dominates over the dwarf character 

 which is suppressed ; and therefore the one is called the 

 dominant character and the other the recessive. From 

 this, and indeed from the whole study of heredity, it 

 follows that we can never judge of the true inheritance 

 or gametic constitution of an individual by mere inspec- 

 tion of its " characters." 



Since the phenomena of inheritance are dealt with in a 

 special volume of this series, we will not attempt to give 

 a complete account of the Mendelian interpretations, but 

 will only summarise some of the main conclusions in so 

 far as they affect the doctrine of evolution in general. 

 It is held that in an organism a number of characters may 

 be distinguished capable of varying independently, and 

 of being isolated or followed separately in breeding ex- 

 periments, and each depending on a special transmissible 

 germinal factor or factors. These characters are known 

 as " unit characters," and the factors which govern them 

 as "unit factors." The whole inheritance would be 

 made up of the sum of its unit factors. No character 

 appears in an individual unless the corresponding factor 



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