780 CONCLUSION xxxn. 13- 



already so familiar as to be almost banal. Thus vertebrates that move in 

 the water tend to have the fish form, with streamlined body, vertebrae 

 with flat articulations and paddle-like limbs. We are so used to this 

 that perhaps its interest is often overlooked, especially the fact that the 

 pectoral limb may revert from an elongated pentadactyl structure to a 

 paddle, sometimes with increased number of digits and of phalanges. 

 Evidently this form of limb is suitable for the uses demanded in the 

 water and tends to be developed again when needed. 



Similar examples that can be called in a sense a reversal of evolution 

 are interesting not only from genetic and embryological points of view, 

 but especially because they show strikingly that under given conditions 

 vertebrate populations tend to react alike, giving us the possibility 

 of developing a reasonable science of morphology. Thus vertebrates 

 that take to the air develop light and thin bones, those that burrow 

 underground often lose their limbs (but among mammals these are 

 often the main digging agents). Similar general evolutionary principles 

 could probably be developed for each organ system. Thus the eyes 

 develop cone-like structures in diurnal, rod-like in nocturnal species, 

 and they become buried under the skin or lost altogether in popula- 

 tions that live in the dark. 



14. Evolution of the whole organization 



For an analysis of the nature of the changes occurring during evolu- 

 tion to be satisfactory it would have to deal not only with the evolution 

 of isolated parts but with that of the whole organization of the activities 

 of the population. In our present ignorance of the morphogenetic 

 processes it is very difficult for us to provide descriptions of the nature 

 of the whole organization that will be satisfactory. We know enough of 

 the underlying hereditary material and the way it produces the chemi- 

 cal action systems and organs of the body to be able to say that there 

 are some genetic factors that affect almost the entire organization, 

 but also that sometimes individual parts change separately. Changes 

 in the activities inherited through the egg and sperm may affect a single 

 process, such as the deposition of pigment in the skin, but usually they 

 influence a wide range of activities and the form of many organs. 

 Conversely each gene is usually influenced in its action by several, 

 perhaps many, others, and the consequent wide range of expression 

 of each character provides the basis of variability on which natural 

 selection can act. 



There are indications that during evolution sets of characters tend to 

 evolve together. For instance, the various features of the mammal-like 



