IN THE THEOEY OF NATURAL SELECTION. 287 



phyletic origin. This first result and meaning of conjugation may 

 be provisionally expressed in the following formula : conjugation 

 originally signified a strengthening of the organism in relation to 

 reproduction, which happened when from some external cause, such 

 as want of oxygen, warmth, or food, the growth of the individual 

 to the extent necessary for reproduction could not take place. 



This explanation must not be regarded as equivalent to that 

 afforded by the theory of rejuvenescence ; for the latter process is said 

 to be necessary for the continuance of reproduction, and ought 

 therefore to occur periodically quite independently of external cir- 

 cumstances ; while according to my theory, conjugation at first 

 only occurred under unfavourable conditions, and assisted the species 

 to overcome such difficulties. 



But whatever the original meaning of conjugation may have 

 been, it seems to have become already subordinated in the higher 

 Protozoa, as is indicated by the changes in the course taken by 

 this process. The higher Protozoa when conjugating do not as a 

 rule coalesce completely and permanently l in the manner followed 

 by the lower Protozoa, and it seems to me possible, or even probable, 

 that in the former the process has already gained the full significance 

 of sexual reproduction, and is to be looked upon as a source of 

 variability. 



Whether this be so or not, I believe it is certain that sexual re- 

 production could not have been entirely abandoned at any period 

 since the time when the Metazoa and Metaphyta first arose ; for they 

 derived this form of reproduction from their unicellular ancestors. 



We know that organs and characters which have persisted 

 through a long series of generations are transmitted with extreme 

 tenacity, even when they have ceased to be of any direct use to 

 their immediate possessors. The rudimentary organs in various 

 animals, and not least in man, afford very strong proofs of the sound- 

 ness of this conclusion. Another example has only recently been 

 discovered in the sixth finger, which has been shown to exist in the 

 human embryo 2 , a part which has only been present in a rudimentary 



1 Coalescence takes place in the so-called bud-like conjugation of Vorticellidae and 

 Trichodinidae, etc. 



s Compare (i) Bardeleben, ' Zur Entwicklung der Fusswurzel,' Sitzungsber. d. Jen. 

 Gesellschaft, Jahrg. 1885, Feb. 6; also ' Verhandl. d. Naturforscherversammlung 

 zu Strassburg,' 1885, p. 203 ; (2) G. Baur, 'Zur Morphologie des Carpus und Tarsus 

 der Wirbelthiere,' Zool. Anzeiger, 1885, pp. 326, 486. 



