296 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



a state of continual oscillation only upon the surface, so to 

 speak, while the fundamental parts of its constitution, which 

 have been inherited from extremely remote periods, remain 

 unaffected. 



Thus sexual reproduction itself did not cease after it had 

 existed in the form of conjugation through innumerable genera- 

 tions of the vast numbers of species which have been included 

 under the Protozoa ; it did not cease even when its original 

 physiological significance had lost its importance, either com- 

 pletely or in part. This process, however, had come to possess 

 a new significance which ensured its continuance, in the 

 enormous advantage conferred on a species by the power of 

 adapting itself to new conditions of life, a power which could 

 only be preserved by means of this method of reproduction. 

 The formation of new species which among the lower Protozoa 

 could be achieved without amphigony, could only be attained 

 by means of this process in the Metazoa and Metaphyta. It 

 was only in this way that hereditary individual differences 

 could arise and persist. It was impossible for amphigony to 

 disappear, for each species in which it was preserved was 

 necessarily superior to those which had lost it, and must have 

 replaced them in the course of time ; for the former alone 

 could adapt itself to the ever-changing conditions of life, and 

 the longer sexual reproduction endured, the more firmly was it 

 necessarily impressed upon the constitution of the species, and 

 the more difficult its disappearance became. 



Sexual reproduction has nevertheless been lost in some 

 cases, although only at first in certain generations. Thus in 

 the Aphidae and in many lower Crustacea, generations with 

 parthenogenetic reproduction alternate with others which re- 

 produce themselves by the sexual method. But in most cases 

 it is clear that this partial loss of amphigony conferred con- 

 siderable advantages upon the species by giving increased 

 capabihties for the maintenance of existence. By means of 

 partial parthenogenesis a much more rapid increase in the 

 number of individuals could be attained in a given time, and 

 this fact is of the highest importance for the peculiar circum- 

 stances under which these species exist. A species of Crus- 

 tacean which inhabits rapidly drying pools, and developes 

 from winter-eggs which have remained dried up in the mud, 



