152 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. 



to changes of their surroundings, and consequently more sub- 

 ject to modification of form and eventual extinction from en- 

 forced migration to unfavorable environment and conditions. 



These unfavorable conditions may result from great cli- 

 matic changes caused by oscillations of the earth's surface, or 

 other cosmic disturbances, which drive out or exterminate the 

 indigenous plants and animals upon which these vertebrates were 

 previously dependent for their sustenance. 



The capacity for migration, however, tends to the survival 

 of these species that possessed it, for "Organisms which are in- 

 capable of moving from place to place in search of food, or of 

 migration to escape vicissitudes of temperature, are much more 

 completely subject to influences of their environment than those 

 that are capable of such movement." 



When, through change of level of the earth's surface, 

 drought has overtaken a region, animals capable of the neces- 

 sary migrations have escaped. A^'Tien an irruption of destructive 

 animal enemies has threatened an animal population with death, 

 those members of it whose strength or speed insured them safety, 

 were the survivors."* 



When we consider the effects of the vast changes in the 

 phvsiographv of the earth described in our former chapter, and 

 the danger from other animals, we can realize something of the 

 terrific struegle for survival among the orehistoric land animals. 

 The effects of these causes are much less perceptible in the 

 case of marine animals, either vertebrate or invertebrate. Hence 

 we find mollusks, crustaceans, radiates, and the lower forms of 

 animal life much more persistent, manv of the genera having 

 come down to us in an unbroken life almost from the first ap- 

 pearance of animal life. 



The Nautilidfe, or Nautilus family, during the Silurian Age, 

 shone with all their lustre, and presented the most varied forms ; 

 likewise manv other families of the Mollusca have come down 

 to us from the same. 



All the srenera of mollusks are not equally plastic, nor rnod- 

 ified by time, the Naticas, Areas, Nuculas, Chitons, Nautilus, 

 etc., have lived during a long-er period than have the o^reat ma- 

 jority of other forms of animal life, and the extinct forms more 

 or less closely resemble the living ones. 



Reptiles' and Mammals do not possess the same resistance 

 to modification, and to this absence of plasticity in the Mollusca 

 is due the fact that, while the moTlusks persist through manv 

 geolosrical aees, the Vertebrates are so subject to change by cir- 

 cumstances that the thinnest stratigraphical horizons can be 

 characterized, from the comparative rapidity of the changes in 



■■ "Origin of the Fittest," by Prof. E, D. Cope, New York, li 



