DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 129 



the nature and value of the protection. Treat similarly the subject of special pro- 

 tective resemblance. Do you know any really harmless animals which assume 

 apparently dangerous attitudes for protection? Accumulate all the available 

 references on mimicry. What range of color have you seen illustrated among 

 animals? In a single animal? Where, on the earth, are the brightest-hued 

 animals found? What are believed to be the causes of colors among animals? 

 What are the uses of colors? What is albinism? Where have you seen instances 

 of it? See Fig. 62. 



FIG. 62. 



FIG. 62. Albino Opossum (Didelphys virginiana). Photo by Folsom. 



Questions on the figure. What is albinism ? In what structures is it manifest ? 

 Among what groups of animals can you find that it occurs? To what is color in 

 hair due? What natural conditions tend to produce color in organisms? What is 

 the chief value of color as an adaptation, individual and social? 



165. Parasitism. Of a nature which combines the quali- 

 ties of commensals and of the preying animals is the association 

 known as parasitism. It is an association of individuals of 

 different species in which one member (the parasite) gets all 

 the benefits, and the other (the host) suffers the loss. It is 

 a case where one species preys on another, but in which it is 

 to the advantage of the parasite, especially if a permanent one, 

 as well as of the host that the life of the latter shall not be 

 suddenly destroyed. It will be readily seen that the parasite 

 increases the work to be done by the host, thus being a handi- 

 cap in the struggle for existence. This might easily bring 

 about the destruction of these species which serve as host were 

 it not for the fact that nearly or quite all animals support vari- 

 ous parasites. Parasites are of two classes, external, as the 

 fleas, lice, and the like; or internal, as most of the parasitic 



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