302 Evolution and Adaptation 
in the chain that connects, without interruption, the older 
inhabitants of the earth with animals living at the present 
time. Without positively affirming that this is the case, he 
did not hesitate to state that a transformation of this sort 
seemed possible to him. He said: “I think that the process 
of respiration constitutes an acquirement so important in the 
‘disposition’ of the forms of animals, that it is not at all 
necessary to suppose that the surrounding respiratory gases 
become modified quickly and in large amount in order that 
the animal may become slowly modified. The prolonged 
action of time would ordinarily suffice, but if combined with 
a cataclysm, the result would be so much the better.” 
He supposed that in the course of time respiration becomes 
difficult and finally impossible as far as certain systems of 
organs are concerned. The necessity then arises and creates 
another arrangement, perfecting or altering the existing struc- 
tures. Modifications, fortunate or fatal, are created which 
through propagation are continued, and which, if fortunate, 
influence all the rest of the organization. But if the modifica- 
tions are injurious to the animals in which they have appeared, 
the animals cease to exist, and are replaced by others having 
a different form, and one suited to the new circumstances. 
The comparison between the stages of development of the 
individual and the evolution of the species was strongly im- 
pressed on the mind of Geoffroy. He says: “We see, each 
year, the spectacle of the transformation in organization from 
one class into another. A batrachian is at first a fish under 
the name of a tadpole, then a reptile (amphibian) under that 
of a frog.” “The development, or the result of the trans- 
formation, is brought about by the combined action of light 
and of oxygen; and the change in the body of the animal 
takes place by the production of new blood-vessels, whose 
development follows the law of the balancing of organs, 
in the sense, that if the circulating fluids precipitate them- 
selves into new channels there remains less in the old 
