No. 3.] FACTORS IN THE EVOLUTION OF MAMMALIA. 393 



the higher sense organs of Petromyzon, which are subject to a 

 remarkable degree of retardation. Thus the eye, throughout the 

 long larval life of the creature, is extremely small and deeply 

 buried beneath opaque tissues of the head. Even at its first 

 appearance the optic vesicles are extremely minute, and the lens 

 is tiny. The retina remains in a rudimentary condition through- 

 out larval life, and only when the larva is metamorphosed into 

 the sexual animal does the eye reach its normal size and develop- 

 ment. Similarly, the olfactory organ during the larval period 

 is a simple epithelial sac, which at the time of metamorphosis 

 becomes an exceedingly complicated structure. The ear alone 

 attains in the larva a degree of development comparable to what 

 it has in the adult. This retarded condition of the higher sense 

 organs has an obvious connection with the habits of life ; for the 

 larva lies buried in the mud and sand of rivers, while the adult 

 lives in clear and swift waters. The effects of the larval habits 

 are shown even in the embryo before hatching ; for the sense 

 organs, and especially the eye, are proportionately much smaller 

 than they are in other vertebrates, even in the earlier stages. 

 These facts are easily enough explained, if we admit the direct 

 action of the environment, at first arising in the larva, and then 

 gradually transferred to the embryonic stages, just in the way 

 in which Hyatt and Wiirtenberger have shown that new charac- 

 ters in the shells of ammonites arise first in the fully adult shell, 

 and then are gradually transferred to earlier and earlier stages. 

 But to refer this retardation in the development of the sense 

 organs of Petromyzon, to what in this connection may fairly be 

 called fortuitous changes in the germ-plasm, is merely to conceal 

 our ignorance under the guise of an explanation. 



When we turn to the series of fossils, and follow out the his- 

 tory of disappearing organs, we find little to support the theory 

 of panmixia. The reduction is steady and sure, if slow, exhibit- 

 ing of course a certain degree of individual variation, but not the 

 fluctuations which we should naturally expect to find, were pan- 

 mixia alone the cause of the reduction. As a matter of fact, 

 when examining an extensive series of fossils reaching through 

 many horizons, it is difficult to escape the suspicion that individ- 

 ual variations are not the material with which natural selection 

 works, so steadily does the series advance toward what seems 

 almost like a predetermined goal. This slowness and steadiness 



