Living Organisms 745 



Illustrations of vestigial structures in other animals are numerous as 

 the following will illustrate: (1) The splint bones of the legs of the horse 

 are remnants of original toes. (2) The poison glands of certain snakes 

 are modified, specialized salivary glands which have evidently developed 

 from the latter through the many stages of descent with change. (3) 

 The gill slits of the embryos (Fig. 363) of higher vertebrates all dis- 

 appear except one pair which develop as the Eustachian tubes connecting 

 the pharynx and the middle ear. (4) The milk glands of mammals are 

 merely modified and specialized sweat glands of the skin. In all prob- 

 ability the various stages of the descent with change brought about this 

 modification. (5) Certain snakes bear small, useless hindlimbs which 

 structurally resemble those of other animals in which they are useful. 



5. Evidences From Comparative Physiology. — Since functions and 

 structures are interdependent, one would expect to find fundamental 

 physiologic similarities in organisms with structural similarities. The 

 following examples will be sufficient to illustrate the point: (1) The 

 bloods of closely related organisms are more nearly alike chemically and 

 physiologically than the bloods of the more distantly related types. (2) 

 The hormones of the excretions of closely related organisms are quite 

 similar and in many instances interchangeable. The insulin of the pan- 

 creas of the sheep may be used for an insulin deficiency in man. (3) The 

 crystal structures of bloods of similar organisms are more nearly alike 

 than the crystalline structures in bloods of more dissimilar or unrelated 

 organisms. In general, common properties persist in bloods of closely 

 related types, and variations are greatest in distantly related forms. 



6. Evidences From Biogeography (Geographic Distribution). — Cer- 

 tain organisms have been observed actually to undergo distinct and spe- 

 cific changes (descent with change) when placed in environments differ- 

 ent from their relatives. 



All the embryos produced by a single female aquatic snail were divided 

 equally into two groups: one group was permitted to develop in the acid 

 waters of a harbor and the other group in the alkaline waters of the open 

 lake. Some time later, the developing offspring were compared. The 

 results were so striking and significant that had the facts not been known, 

 one would have stated that there was no relationship between the groups. 

 Those developing in the acid waters had very thin, semitransparent 

 shells; those in the alkaline water had large, heavy, crusty shells. Is this 

 not suggestive of what happens in other living organisms all the time? 

 May forms originally closely related develop along entirely different 



