AND THE WEST COASTS OF THE UNITED STATES. 131 



b. The individuals in their development have been subjected to diverse condi- 

 tions. The importance of such diverse conditions upon form is very great. Every 

 external factor has its modifying influence on development. Just because the 

 chemical, osmotic, aqueous, molar, gravitational, photic, and thermic agents do not 

 act at all times to the same degree and in the same direction upon all the individuals, 

 the individuals in any generation are unlike. 



c. The individual shells have not exactly the same ancestry. It is quite probable 

 that some contain the blood of incipient races that the others do not. When the 

 form-unit was nascent, at least, some degree of intermingling with adjacent forms 

 probably occurred, but this did not affect all individuals alike. Even when the 

 commingling forms did not belong to distinct races or species, but were only "sports," 

 their influence would be felt through remote generations. 



2. Two lots may have unlike variability partly because causes a, b, and c may 

 have acted unequally upon them and partly also because in one locality different 

 selective factors may have been at work from those acting in a second locality. There 

 is some evidence (Bumpus, '98) that where selection acts most rigidly variability 

 is most diminished. Finally, unlike variability may arise as a result of changing 

 environment whereby a preferential selection of the more plastic — the more accom- 

 modating, i.e., the more variable (Pfeffer, '94; Baldwin, '97) — may take place. 



To judge which factors have brought about the diverse variability of the Tampa 

 and San Diego forms we need to consider more fully the geographical histories of the 

 Tampa and the San Diego communities. 



3. The Dunedin Pectens are inhabitants of the mud-flats and shallow waters of 

 the Gulf of Mexico. The shore is of old Miocene date and contains no overlying 

 Pliocene deposits, according to Dall and Harris ('92). It is only 25 miles from the 

 mud-flats that surrounded the "Eocene Island" of Florida. Although this shore-line 

 has doubtless experienced some fluctuations, on the whole it has long remained such 

 a mud-flat as it is to-day. 



4. Unlike southern Florida the coast of southern California is one of bold 

 relief. Mud-flats are few and consequently the places where our Pecten can 

 develop are far between. But these conditions have not always existed in, geologic- 

 ally speaking, rather recent times. Elevated sea-cliffs in which Pectens indis- 

 tinguishable from the living species are found afford indisputable evidence that 

 the whole coast about San Diego was 800 to 1500 feet lower than it is now, 

 and that the depression occurred during Pliocene (pre-glacial) time. Lawson ('93) 

 states that "as a consequence of the general uplift of the coast the physiography 

 of the country has been radically changed in the most recent geological times. 



