336 Cell as Basis of Organic Activity 



ichthyosaurs, and the mammalian whales, dolphins, and porpoises. To 

 a lesser degree, streamlining is seen among some diving birds and 



seals. 



Nowhere are the contrasting principles of divergence and con- 

 vergence more clearly illustrated than in the parallel evolution of the 

 Australian marsupials (mammals carrying their young in a pouch) and 

 the placentals of other continents. In both cases there has been tre- 

 mendous divergence, resulting in animals fitted for the occupation of all 

 habitats. There are both marsupial and placental wolves, marsupial and 

 placental moles, and marsupial and placental cats. The flying squirrel 

 has its counterpart in the marsupial phalanger. Those forms occupying 

 similar habitats thus show remarkable likeness of form and habit. 



Mimicry and Protective Coloration. — At times the success of 

 an animal in any particular environment is due to some rather peculiar 

 adaptations. Protective coloration and mimicry are examples of some 

 of these types. 



Finding an animal in his natural surroundings is often extremely 

 difficult. A tiger in the zoo is a conspicuous creature, but in his native 

 habitat, his stripes blend into the broken shadows. Among smaller ani- 

 mals, this resemblance to background is at times startling. The horned 

 lizard with his spiny skin looks like a flattened stone buried in the dust. 

 Among insects, such animals as the walking stick not only look like 

 a dead stick but even orient their bodies in a sticklike manner. Many 

 caterpillars look like dead bits of leaves. Some spiders look like 

 thorns, and a few resemble bird droppings. A few butterflies when 

 at rest resemble leaves so closely that even the pattern of the leaf 

 veins is visible. In fact it can be stated that with but few exceptions 

 an animal has little chance of survival unless it does in some manner 

 blend into the background in which it lives. Exception to this is dem- 

 onstrated by an animal such as the skunk which has a conspicuous 

 warning coloration ; this serves to drive away would-be predators. 



Further development of this concealing pattern is found in some 

 animals that mimic or resemble other forms. The most familiar ex- 

 ample of this is the viceroy butterfly which closely resembles the mon- 

 arch. The conspicuous monarch has been shown to be distasteful to 

 birds, and perhaps the tastier viceroy gains immunity from predators 

 by its resemblance. Many kinds of insects and spiders live in ant 

 colonies. This gives them protection and also a ready food supply. 

 Many of these strange forms known as myrmecophiles resemble their 



