96 



The Outline of Science 



of the trumpeting with which frogs herald the spring, and it is 

 often only in the males that the voice is well developed. But if 

 we look forward, past Amphibians altogether, we find the voice 

 becoming a maternal call helping to secure the safety of the 

 ing a use very obvious when young birds squat motionless 

 at the sound of the parent's danger-note. Later on, probably, 

 the voice became an infantile call, as when the unhatched croco- 

 dile pipes from within the deeply buried egg, signalling to the 

 mother that it is time to be unearthed. Higher still the voice 

 expresses emotion, as in the song of birds, often outside the limits 

 of the breeding time. Later still, particular sounds become words, 

 signifying particular things or feelings, such as "food," "dan- 

 ger," "home," "anger," and "joy." Finally words become a 

 medium of social intercourse and as symbols help to make it 

 possible for man to reason. 



2 

 The Early Reptiles 



In the Permian period reptiles appeared, or perhaps one 

 should say, began to assert themselves. That is to say, there was 

 an emergence of backboned animals \vhich were free from water 

 and relinquished the method of breathing by gills, which Amphib- 

 ians retained in their young stages at least. The unhatched or 

 unborn reptile breathes by means of a vascular hood spread un- 

 derneath the eggshell and absorbing dry air from without. It is 

 an interesting point that this vascular hood, called the allantois, 

 is represented in the Amphibians by an unimportant bladder 

 growing out from the hind end of the food-canal. A great step 

 in evolution was implied in the origin of this antenatal hood or 

 foetal membrane and another one of protective significance- 

 called the amnion, which forms a water-bag over the delicate 

 embryo. The step meant total emancipation from the water and 

 from gill-breathing, and the two foetal membranes, the amnion 

 and the allantois, persist not only in all reptiles but in birds and 



