12 GENERAL ZOOLOGY 



division of the entire animal into halves, each of which 

 then grows to the original size. In multicellular animals, this 

 same simple method may take place, but only in lower forms. 

 The method of reproduction common in all forms above the 

 Protozoa is far more complicated and its discussion must be 

 postponed until a later period. Suffice it to say, that the ele- 

 ments of the reproductive process are an egg cell, produced by 

 the female and a sperm cell produced by the male. These unite 

 in fertilization and out of this relatively simple beginning the 

 organism gradually develops into the adult form. Each stage of 

 development is alive. The different stages represent the organ- 

 ism in stages of transformation. Under some conditions the 

 egg alone may reproduce the adult animal, showing that at least 

 in some forms the potency for reproduction is possessed by the 

 egg alone. 



Vitalism and Mechanism. — The list of attributes of proto- 

 plasm as outlined above constitute a definition of an organism. 

 Can these attributes be explained in terms of the protoplasm 

 doctrine or is it necessary to assume the presence of some super- 

 natural, vital factor in addition to known chemical and physical 

 factors? Formerly, the idea that the living body is presided over 

 or directed by some kind of vital force or energy such as the 

 "soul" of Descartes or the "entelechy" of Driesch met with 

 more general acceptance than at the present time. The pro- 

 ponents of such a view are usually spoken of as "vitalists," and 

 their doctrine is known as vitalism. According to the vitalists, a 

 living thing is protoplasm plus some unknown something that is 

 absent in nonliving objects. The unknown thing is called the 

 vital factor, without which life cannot exist. The objection to 

 such a view from the scientist's standpoint is that it bars the way 

 to investigation. If investigators of living matter must always 

 admit the presence of a factor which cannot be seen, felt, 

 weighed or measured, that is to say, a factor that cannot be con- 

 trolled, his attempt to solve the riddle of life in scientific terms 

 is doomed to failure from the start. Such a view discourages 

 investigation and creates an atmosphere of hopelessness which is 

 not conducive to success. It does not help but actually hinders 

 further investigation into the problem. For this and other 

 reasons many biologists are inclined to support the protoplasm 

 doctrine which is a mechanistic theory of life to the extent that it 



