DR. THEODOR NEUMANN. 217 



duces its species has nothing arbitrary or accidental, 

 but is influenced in a very regular fixed way by the de- 

 velopment which its ancestors had to undergo. All those 

 changes v^'^hich occurred in the course of former genera- 

 tions, the result of which is shown in the organism in 

 its shape to da3% are repeated in the development of the 

 individual. The oldest and simplest forms of life which 

 we know are organisms consisting of one cell only; 

 all the animals now living, however large and compli- 

 cated in their structure, originate from an egg, one single 

 cell. The oldest vertebrates were water-animals breath- 

 ing through gills; in the earliest stages of development 

 of any vertebrate there is a stage where indeed not the 

 gills themselves, but parts of the skeleton bearing them, 

 are apparent. Mammals and birds have developed from 

 reptile-like ancestors; and thus their embryonal devel- 

 opment contains even now phases which can not be dis- 

 tinguished from each other. All these are features which 

 disclose to us a common origin. In one word, in its de- 

 velopment each individual reproduces, as it were in 

 outline, the different stages in the gradual metamorphosis 

 of the animal during the series of generations of its an- 

 cestors. If we, therefore, know the history of the de- 

 velopment of one animal, the stages through which it 

 passes during its creation, or its ontogeny, we possess, 

 consequently, nearly always data for the determination 

 of its ancestors and its relatives; we obtain light on the 

 various changes which the ancestors of the animal had 

 to go through until they arrived on the present stand- 

 point: in other words, we can trace its phylogeny. 



Such changes are usually of a twofold character— either 

 degenerative, regressive, or progressive; that means, the 

 parasites lose certain characteristics of their structure 

 and obtain others instead. We have stated that tem- 

 porary parasites are in form and shape essentially equal 

 to the free-living animals. They must have, like the 



155 



