PROBLEM 4. 



Theories about the Origin of Kinds of Living Things 561 



loss of water. Mistletoe has a rootlike 

 structure that grows through the bark 

 of trees enabling it to get water and 

 minerals. The grape and many other 

 vines (Figure 280, page 302) have ten- 

 drils which hold the stems upright. Some 

 plants, such as most lichens (Figure 326, 

 page 375) remain alive even though they 

 may become so dry they can be crushed 

 to a powder. 



These adaptations can be reasonably 

 explained by assuming that they are the 

 result of mutations which made the mu- 

 tants especially well fitted to get along 

 in their environment. 



Evolution and how it might occur. The 

 theory of evolution is a complex idea. 

 It states that all living things are related 

 to one another because they all came 

 from the same common ancestors far 

 back in geologic time. It states that new 

 species arose from preceding species of 

 plants and animals that were simpler, 

 that this has been happening since plants 

 and animals first existed on this earth, 

 and that it is still going on. This state- 

 ment of the theory of organic evolution 

 is commonly accepted by biologists. 



But all biologists do not agree on a 

 theory that can explain hoiv this hap- 

 pened. Most biologists today accept De 

 Vries's theory or modifications of it. 

 This is really Darwin's theory with the 

 idea of mutations and chromosomal 

 changes substituted for the general idea 

 of variations. If you use the five steps 

 of Darwin's theory of natural selec- 

 tion, substituting mutations (germinal 

 changes) for the step "variation," you 

 will have a clear statement of De Vries's 

 theory of the origin of species by mu- 

 tation. 



Fig. 512 TJ}is insect is called the walking stick. 

 Why? Can you see how its structure helps it 

 to hide fro7n its enemies? (schneider and 

 Schwartz) 



which is one form of protective adapta- 

 tion. Figures 5 1 1 and 5 1 2 illustrate 

 striking examples of protective resem- 

 blance in insects. Figure 510 is an ex- 

 ample of protective resemblance in ver- 

 tebrates. Other animals have special 

 structures which fit them to their en- 

 vironment in difl^erent ways. The ich- 

 neumon fly can place its eggs in the 

 body of a caterpillar buried an inch 

 deep in wood because it has a long and 

 very strong ovipositor or egg-laying or- 

 gan. Moles have large, strong front legs 

 and feet that enable them to burrow 

 rapidly through the soil. You can think 

 of many other examples. 



Many plants, likewise, have adapta- 

 tions useful to them. Cactus plants liv- 

 ing in the desert have a thick epidermis 

 and a thick cuticle that help prevent 



