368 



THE CHANGING GENERATIONS 



within an egg shell (Fig. 28.2). Obviously neither their ancestors nor 

 themselves as adults could possess such structures. These are special 

 embryonic organs, superimposed on the inherited ontogeny. Many other 

 organisms become free-living in early stages of development. Such larvae, 

 as they are called, must make their own living while they complete their 

 development. Often they are enabled to live in environments unlike those 

 inhabited either by the ancestors or by themselves as adults, through 

 having special larval adaptations. Among many insects the larval struc- 

 ture has become so different from that of the adult that the change from 

 one to the other is known as metamorphosis, or transformation. It is 



accomplished by the interpolation 

 of a pupal stage into the develop- 

 mental process. The larval body 

 becomes enclosed in a protective 

 case, the pupa, within which the 

 entire internal and external organi- 

 zation of the larva is rapidly 

 reworked into that characteristic 

 of the adult. 



Such modifications of the develop- 

 mental stages as these may be 

 thought of as things added to or lost 

 from the original ontogeny. They 

 often so alter and distort the pattern 

 of development that the evidences 

 of recapitulation become difficult 

 to detect and interpret. 



Physiological resemblances. 



Fig 24.10. Life history of the striped cu- 

 cumber beetle (Diabrotica vittata). A, stem- 

 boring larva. B, pupa. C, leaf-feeding adult. 

 This illustrates complete metamorphosis; 

 the great differences between larva and 

 adult are bridged over by the pupal stage, 

 in which the anatomy is rapidly reworked. 

 In such a developmental history as this 

 the evidences of recapitulation are very 

 much obscured and distorted. (Modified 

 from Turtox chart, courtesy General Bio- 

 logical Supply House, Inc.) 



Kinship is as clearly reflected in the 

 physiology and biochemistry of organisms as in their structure and 

 ontogeny. The well-nigh universal presence of chlorophyll in plants and 

 of the process of photosynthesis dependent upon this substance is clearly 

 a case of homology based upon common descent. Similarly we find in all 

 vertebrates (and only in them) a type of respiratory-circulatory process 

 dependent upon the presence of blood cells containing hemoglobin. In 

 both these instances the portions of the gene complex responsible for the 

 respective physiologic processes must be of great antiquity, since so 

 many different kinds of organisms possess them. 



Not all instances of physiological resemblance are homologies, however. 

 Thus hemoglobin, present in the red blood cells of all vertebrates, also 

 occurs in a few species of annelid worms and insects. This is clearly a case 

 of physiological analogy, for the hemoglobin of these invertebrates differs 

 chemically from any vertebrate hemoglobin and is diffused in the blood 



