106 INTRODUCTION TO EVOLUTION 



proportion of nitrogenous wastes excreted as ammonia has increased, as 

 noted previously). The changes in both visual pigments and nitrogen ex- 

 cretion may be adaptations to the resumed aquatic life. 



In some cases the environment in which the animal was spawned seems 

 to determine which substance shall be present or predominate. Thus 

 fresh-water eels spend most of their lives in fresh water but migrate into 

 the sea to spawn. They have both substances but the rhodopsin predomi- 

 nates. At the time of migration from fresh water to the sea, in fact, all 

 detectable pigment may be rhodopsin. Salmon also possess both sub- 

 stances, but in their case the porphyropsin predominates. They spend most 

 of their lives in the sea but spawn in fresh water. 



Bullfrogs were found by Wald to exhibit an interesting recapitulation of 

 the change from porphyropsin in fresh-water fishes to rhodopsin in their 

 descendants, the amphibians. Bullfrog tadpoles possess the porphyropsin 

 system, with only a trace of rhodopsin. Thus, living in fresh water, they 

 resemble fresh-water fishes. At the time of metamorphosis from tadpole 

 to adult the eyes develop the rhodopsin characteristic of land vertebrates. 

 Here we have another instance of biochemical recapitulation, although 

 there may be nothing mysterious about it beyond the mystery of why por- 

 phyropsin should be adaptive for life in fresh water, rhodopsin for life 

 in the air. Still other cases were given by Wald (1958) who stressed the 

 point that the change in visual pigment is only one of several biochemical 

 changes undergone during metamorphosis. Further research may reveal 

 that the chemical change in visual pigment is only a more or less inci- 

 dental accompaniment of more profound biochemical changes under- 

 gone by animals as they adapt to fife in fresh water, on the one hand, or 

 to life in the ocean or on land, on the other. (Yet there are a few 

 wholly marine fishes that have porphyropsin!) Obviously we must await 

 the results of further research before making any sweeping generalizations 

 about the adaptive significance of these substances. 



Conclusion 



In this chapter we have discussed some of the fundamental similarities 

 (homologies) exhibited in the chemical structure and metabolic processes 

 of living things. We have also considered some of the evolutionary 

 changes which have modified basic patterns as organisms have become 

 adapted to diff'ering environments. We see that adults and embryos have 

 changed during their evolutionary histories not only in morphology but 

 also in physiology. The fact that our knowledge of the morphological 



