
1891.] Quantity and Dynamics of Animal Tissues. 979 
Brachiopoda, as Discina and Productus, a calcareous shell made 
up a large part of their weight. The rugose and tabulate corals, 
the Cephalopods, and many other forms were permeated through 
and through with walls and tables of calcareous masonry. This 
process continued for a long succession of ages. Even when the 
early fishes appeared they too were plated and shielded by thick 
secretions of carbonate of lime. But as the type of fishes 
expanded a new tendency became developed, which has con- 
tinued to manifest itself ever since. Even the early Ganoids pos- 
sessed a much greater quantity of soft tissue in proportion to the 
hard parts than did the Brachiopods and ‘corals. This, of course, 
was an indispensable prerequisite to the free and roving life which 
they led. Had they been weighted as heavily as some of the 
Brachiopods, locomotion would have been out of the question ; in 
fact, it may be enunciated as a general principle that wherever an 
animal is found capable of vigorous and long-continued locomo- 
tion, there we may be sure to find the motile elements of the body 
sustaining a high ratio to the inert elements. But later on forms 
like the Cestracionts and other sharks must have hada very large 
excess of soft parts over hard parts. Later still the heavy ganoid 
scales were discarded for the lighter ones of the Teleosts. The 
secretions of lime became less, and the excess of hard parts over ` 
soft parts was diminished until the opposite relation existed. 
While this tendency may not be traceable in all cases, yet there 
are a sufficient number of instances to warrant the generalization 
that the development of animal life has been marked by a grow- 
ing diminution in the quantity of hard parts, and a relative 
increase in the quantity of soft parts. The gigantic reptiles of the 
Cretaceous age and mammalian life of later times all point to this 
conclusion. The change in the relative quantities of hard and 
soft parts was attended by other correlated changes. As the soft 
parts were liberated from the incubus of the hard parts they 
gradually differentiated in various directions, to suit the altered 
conditions of their environment. By reducing its calcareous _ 
integument the animal was more exposed to the attacks of its 
enemies ; and this exposure necessitated one of two things,—either 
it must take on protective colors or spines, or it must differentiate 



