IOWA ACADEMY OF SCIENCES. 27 



2. During its development it passes through a series of 

 successive stages similar to those through which its parent 

 passes at corresponding periods, and finally on reaching 

 maturity resembles that parent closely in form and func- 

 tion; indeed, is identical with it, except in those minor 

 characters called individual. 



3. The direction of development must be determined 

 by either one, or both, of two sets of conditions: the 

 first iiiternal, namely, the structure and resident forces of 

 the ovLim given to it by the parent; the second external, 

 the matter and forces reaching it from without. The 

 internal sets of conditions we call heredity ; the external, 

 the environment. 



These two factors are constantly co-operating in every 

 developing organism; but, as we shall see, the relative 

 importance assigned to each by different biologists varies 

 enormously. 



To explain the operation of the first factor, heredity, 

 many hypotheses have been proposed. These may be 

 divided more or less completely into two groups, preform- 

 ism and epigenesis. 



Theories of preformation suppose that in the germ are 

 represented in some way all the various parts of which 

 the adult is composed. Development then consists merely 

 in the growth and unfolding of these parts. The germ is 

 consequently of a complexity comparable to that of the 

 adult, its apparent simplicity of structure being due to our 

 inability to distinguish the parts present on account of 

 their minuteness. Theories of epigenesis suppose that the 

 germ is realty of a relatively simple structure, and that 

 development consists in the actual addition of new parts 

 in an orderly manner under the control of the parts previ- 

 ously present and the forces of the environment. 



For the beginning of preformation theories we must go 

 to the ancient Greeks. Democritus, about 400 B. C, sup- 

 posed that the germ was built of very minute corpuscles 

 derived from all parts of the body of the parent, these 

 corpuscles having the power to grow enormously, each at 

 its proper time, and so reproduce the part from which it 



