362 



PRINCIPLES OF ANIMAL BIOLOGY^ 



Orthogenesis, if there is such a phenomenon, must not be regarded 

 as an abstruse perfecting principle of any kind, but as being dependent 

 on the chemical composition of the protoplasm. It is therefore to be 

 expected that orthogenesis may occur in some animals, not in others. 



Evolution by Hybridization. — Although new characters probably 

 arise regularly in higher animals only by mutation, it is obvious that a 

 form of evolution may take place by combination, in one organism, of 

 features that have previously existed independently in different organisms. 

 They are brought together in one individual by hybridization, producing 

 combinations that did not exist before; for example, rough coat in guinea- 



FiG. 245. — Combs in fowls. A, pea comb; B, rose comb; C, single comb; D, walnut 

 comb. The walnut comb is a compound character, that is, it is produced by the coopera- 

 tion of the factors for pea comb and rose comb. (From Punnctt's Mendelism Courtesy of 

 Macmillan Co.) 



pigs may be combined with any of the coat colors. The combination 

 is not always so simple as this, however, even where only two characters 

 are involved. The bringing together in one animal of genes for two 

 characters not previously combined sometimes produces a new char- 

 acter unlike either of those combined. Fowls with pea combs (small 

 combs with a number of low prominences, Fig. 245), breed true to their 

 form of comb. Rose comb (a low but thick comb with a roughened 

 upper surface and a long tapering projection behind) is another true- 

 breeding character. When a fowl homozj^gous for pea comb is mated 

 with one homozygous for rose comb, the result is not a fowl having both 

 pea and rose combs, but a new comb called walnut. This is a thick, 

 fleshy, rounded comb overhanging the base of the beak. Other in- 

 herited features, requiring for their production the presence of half 

 a dozen or more genes for simple characters, are known. The results 



