294 A N INTRODUCTION TO ZOOLOGY 



1 60. They differ widely from one another in structure and habits 

 but in many cases may be traced back to a single ancestor, the 

 blue-rock pigeon (Columba livia), still in existence in its wild state 

 (Fig. 1 60, i). The theory is that these pigeons have been origi- 

 nated by the preservation for breeding purposes of birds that 

 showed favorable variations. 



Now animals vary in a wild state as well as under the care of 

 man, and whenever an individual appears with a variation of 

 value in the struggle for existence, this particular animal becomes 

 one of the fittest to survive, and, therefore, lives to transmit its 

 favorable variation to its offspring. In other words, it is selected 

 by nature just as a domesticated animal with a favorable varia- 

 tion is selected by Man. As an example, we may take the robin. 

 We have shown (p. 283) that this bird rears a number of young 

 each year, yet the number of robins remains practically constant 

 from year to year. The majority are killed in some way. Those 

 that are strongest and are able to weather the storms, or are 

 endowed with extreme speed so as to escape their enemies, are 

 selected by Nature, while weaker or slower individuals are 

 doomed to destruction. 



Animals become adapted to their surroundings through the 

 operation of this process of natural selection. The wonderfully 

 adapted structures on the legs of the honeybee for gathering 

 pollen may have evolved in this way. 



Of recent years many biologists have questioned the adequacy 

 of natural selection to explain all the modifications of species. 

 This was never claimed for the theory, even by Darwin, for he 

 says, in the introduction to the Origin of Species, " I am convinced 

 that natural selection has been the main, but not the exclusive, 

 means of modification " (236). 



THE THEORY OF ORTHOGENESIS. Many phenomena of the ev- 

 olution of organisms cannot be explained by natural selection. For 

 example, the development of structures of apparently no advan- 

 tage to the animal, such as the arrangement of the veins in the 

 wings of insects, and the development of modifications harmful 



