GEOLOGY: /. BARRELL 
503 
critical factor in their environment. No traces of amphibians are found, 
though by the law of anticipation they must have been in existence in 
Upper Old Red Sandstone times; since they are typically developed 
at the opening of the following period. The amphibians seem therefore 
to have originated in more limited regions where fishes could not sur- 
vive and the waters were not sufficiently permanent for the preservation 
of their remains. 
The preceding arguments have been drawn from the geologic record. 
Let us now turn to another record, — that embalmed in the nature of 
living vertebrates, especially lung fishes and amphibians; for they pre- 
serve practically unaltered a stage of respiratory and circulatory evo- 
lution which had been attained in the Devonian. We may find by ex- 
amining their organic nature whether the development of lungs was a 
spontaneous, internally directed organic advance, giving increased 
activity and efficiency, or whether at first it was a mere makeshift 
response to the pressure of adverse external conditions. 
The air-bladder of ganoids and higher fishes is related to the intestinal 
region, not the pharyngeal region, as is shown both by its direct con- 
nection with the oesophagus in ganoids and in the embryos of other 
fishes. The blood system shows the same, the dipnoans, ganoids and 
amphibians having the air-bladder or lungs supplied with blood from a 
branch of the fourth efferent gill artery. In each round of the circulation 
only a portion of the blood is passed through the primitive lungs. The 
efiiciency of this is so limited that the higher fishes living in well-aerated 
waters, though descendents of ganoids, have turned a primitive breath- 
ing organ to other uses and have reverted wholly to the use of gills. 
The function of the air-bladder was, then, to tide over periods of par- 
tial asphyxiation in foul waters, and seems to have been initially de- 
veloped from a habit of swallowing air, gasping and finding this mode 
of relief. If forced to rely wholly upon this, it sufficed merely to keep 
the fishes alive until a new season of floods should recur, expanding the 
sphere of action and the supply of food simultaneously with the renewed 
gill-respiration and increased activity of the inhabitants of the river and 
playa waters. The use of gills, furthermore, could not become com- 
pletely eliminated until such great changes had progressed in the heart 
and circulation inherited from elasmobranchs as are found in modern 
lung fishes and amphibians. It was this initial inefficiency of air- 
breathing which seems the strongest proof that it was an adaptation 
forced by the compulsion of nature, not the expression of a hypothetical 
innate tendency for advancement in organization. Of course, the or- 
