EVOLUTION OF THE COLORS OF BIRDS. 79 



6. Bilateral symmetry. It is not clearly apparent why 

 bilateral symmetry should be so prevalent in organic 

 structures, whether it be due to some fundamental law 

 of growth, or persistence of type, or to some principle 

 of utility. Wallace has advocated the last view with re- 

 gard to the bilateral symmetry in the colors of animals, 

 arguing from the fact that where protective or recogni- 

 tion markings are no longer of utility, as with domesti- 

 cated animals, bilateral symmetry is lost. 



7. Correlation of groivth. This law has been most 

 clearly enunciated and established by Darwin. Owing 

 to the close interdependence of parts in organisms, any- 

 thing which affects one structure in the body may cause 

 a similar or corresponding modification to appear in 

 some other structure which is not directly influenced. 



The laws of heredity may be condensed after Haeckel, 

 thus:* 



1. Uninterrupted or continuous transmission. Chil- 

 dren are in general like their parents. This is expressed 

 by the phrase 'Mike produces like," or more accurately 

 " similar things produce similar things." 



2. Interrupted or latent transmission. Among certain 

 lower forms of life there is an alternation of generations, 

 the children being like the grandparents, the parents a 

 different organism. "If we express this general law 

 and the succession of generations by the letters of the 

 alphabet, then A= C = E, whilst B = D = F, and so on." 



3. Sexual transmission. Each sex transmits to its 

 offspring certain peculiarities not possessed by the other 

 sex, as, for example, the antlers of a deer. 



4. Mixed or mutual (amphigonous) transmission. 

 "This law tells us that every organic individual pro- 

 duced in a sexual way receives qualities from both 

 parents, from the father as well as from the mother." 



* History of Creatiou, pp. 205-213. 



