162 



POPULATIONS OF THE SEA 



Fig. 4. Numbers and individual survi\"al prospects of marine organisms 

 regulated by different internal and cobiotic factors. 



parental dependency, and degree of parental care in general 

 increase toward the left, with decreasing size of the breeding 

 population. Competitive factors are involved throughout. More- 

 over, an important paleobiogeographical exception to the direction 

 of size increase is provided in the case of the shelled invertebrates. 

 Other factors being equal, they grow larger in the presence of an 

 abundant and regular supply of calcium carbonate, or in the 

 warmer, normally salty seas of any given plane in time. 



There is, in any event, a relationship between mode of life, size 

 of breeding population, number of offspring, and individual 

 survival prospects which is important to the paleobiogeographer. 

 But survival of the individual is only one of the factors that 

 controls survival of the species, and there is probably an optimum 

 size to breeding populations w^hich varies in the same direction as 

 numbers of offspring and is related in some way to mutation 

 incidence. The paleobiogeographical evidence, moreover, is seri- 

 ously affected by the ability of a species to survive in the rocks as 

 well as in time. Because of this, paleontologists concentrate on 

 organisms with preservable hard parts, on special types of deposits 

 where the imprints or remains of soft-bodied organisms may be 



