n8 THE SHAPES AND SIZES OF ANIMALS 



resemble one another, or why birds should be con- 

 structed on one common plan. If then it is not mere 

 matter of chance, can we determine in any way 

 what are the causes which govern the shapes of 

 animals, and what are the laws in accordance with 

 with which their effects are produced ? 



So with size : for each animal we have a certain 

 standard of size, which is rarely very greatly 

 departed from. Such names as cat, pigeon, cock- 

 roach, convey to our minds not merely impressions 

 of animals of certain shape, structure, and 

 habits, but also of tolerably fixed dimensions. 

 A cockroach as big as a cat would at once arrest 

 our attention as unusual. The problem we have 

 to deal with is to find out, if possible, the causes 

 which regulate the dimensions of animals, and 

 determine that there shall be for each kind of 

 animal a certain average size. These are most 

 elementary considerations, but they will serve to 

 show us what our subject is, and that it is one well 

 worthy of attention. 



With regard to the shapes of animals, we find 

 that among the simplest animals, or Protozoa, it is 

 characteristic of the more primitive genera that 

 there should be no definite or consistent shape. 

 This is well seen in Amceba, which consists of a 

 minute speck of protoplasm equally contractile 

 in all directions ; protrusions of the protoplasm, 

 known as pseudopodia, being put out from any 

 part of the surface, and in any direction. In 

 Amoeba and in all forms which exhibit similar 

 " amoeboid " movements, there is no distinction of 



