Evolution 337 



hosts rather closely. Inasmuch as ants are very sensitive to odors and 

 textures, the resemblance of these often unwanted and even destructive 

 guests must be even greater than appearance alone. 



The development of these most unusual resemblances is difficult to 

 explain. It can only be conjectured that random mutations and the 

 environmental selection have favored their development. Again it must 

 be emphasized that there has been sufficient time for even these im- 

 probable resemblances to develop. 



THE DISTRIBUTION OF ANIMALS 



Lyell, the Scottish geologist, was the first to point out that the 

 present-day distribution of animals is the result of past geological 

 history. This concept impressed Darwin and was one of the important 

 influences affecting the later development of his theories. The truth 

 of Lyell's theory has been shown repeatedly as knowledge of the dis- 

 tribution of plants and animals has increased and as paleontological ma- 

 terial has accumulated. 



It is now recognized that animal groups originate in definite cen- 

 ters. Due to population pressures and competition for food, the more 

 vigorous individuals or forms tend to spread out from the periphery in 

 all directions. This spread continues until stopped by some physical 

 or other barrier. 



Migration. — From evidence of both geological and present dis- 

 tribution, it is clear that certain definite migration routes have been 

 favored in the spread of animals and plants to other regions. One of 

 these much used routes was that across the Bering Strait. This has 

 resulted in a strong similarity between the animals and plants of 

 northern Eurasia and North America. Another important pathway was 

 that through Central America. Through this corridor groups originat- 

 ing in North America spread into South America, and some few forms 

 spread in the opposite direction. These pathways were not open through 

 all geological eras. The Central American one closed and opened many 

 times, influencing the present-day distribution of many different forms. 



Barriers. — Any vigorous animal will spread until some barrier 

 impedes further migration. These barriers may be of several types. 

 Often they are actually physical ones such as bodies of water, mountains, 

 and deserts. Physiological barriers such as unfavorable temperatures 

 or humidity relationships may exist. Again it may be competition 



