Vol. 8, 1922 
BIOLOGY: A. H. CLARK 
219 
9 Pearl, R. and Parker, S. L. Amer. Nat. 55, pp. 481-509, 1921. 
10 Pearl, R. Genetics. 2, pp. 417-432, 1917. 
11 Pearl, R. and Surface, F. M. U. S. Dept. Agr. Bur. Amer. Ind. Bulletin No. 110, 
Part 1, pp. 1-80, 1909. 
ANIMAL EVOLUTION 
By Austin H. Clark 
U. S. National Museum, Washington, D. C. 
Read before the Academy, April 24, 1922 
There are few lines of scientific speculation of such general interest as 
the problem of the evolution of the varied assemblage of types included 
in the animal kingom, while at the same time a logical and detached con- 
sideration of this problem is rendered exceedingly difficult both because of 
the isolated position occupied by many of these types and by the convic- 
tion we all have that man must represent the highest of them. 
But while undoubtedly man is the most efficient and the dominant mem- 
ber of the animal kingdom it by no means necessarily follows that he and 
his fellow vertebrates are from the strictly biological standpoint the most 
perfect. 
In the following pages I shall indicate, as briefly as possible, a line of 
reasoning whereby all the various animal types are brought into correla- 
tion with each other, and their evolution is shown to be not evolution in 
the sense of a progressive development from a lower type to a higher, but 
instead the gradual acquisition of increasing economic efficiency through 
the progressively greater and greater departure from biological perfection, 
correlated with the gradual loosening of the bonds by which the most 
perfect type is economically handicapped. 
In a very large and important group of animals, the Protozoa, the body 
is composed of a single cell; but in all animals except the Protozoa the 
body is composed of a very large number of cells which are differentiated 
to serve definite purposes. 
All multicellular animals begin life as a single cell. As at this stage they 
are in this respect comparable in structure to a protozoan, in later life 
they may be assumed to represent an advance over the protozoan type. 
The original cell giving rise to a multicellular animal typically divides 
into two, four, and eight similar cells, the cleavage taking place in three 
planes each at right angles to the other two; each of these eight similar 
cells continues to divide until a hollow ball is formed of numerous cells all 
of which are alike. Such a structure is represented by a blastula, but no 
adult animal (? except Salinella) which can be regarded as the equivalent 
of such a stage is known. 
