ORGANIC EVOLUTION— PHYSICAL 37 



organisms only conjugate at rare intervals ? the answer 

 may be given in the following extract — 



" It has often been alleged that the subsequent divid- 

 ing is accelerated by conjugation, but Maupas finds that 

 this is by no means the case. The reverse in fact is 

 true. While a pair of infusorians were engaged in con- 

 jugation, a single individual had, by ordinary asexual 

 division, given rise to a family of from forty to tifty thou- 

 sand individuals. Moreover, the intense internal change 

 preparatory to fertilization, and the general inertia 

 during subsequent reconstruction, not only involved loss 

 of time, but exposed the infusorians to great risk. Con- 

 jugation seems to involve danger and death rather than 

 to conduce to multiplication and birth." — Thompson, 

 Elements of Zoology. 



The above explains also why sexual reproduction 

 does not occur in all instances, e. g. when the specific 

 persistence is secured by extreme rapidity of multiplica- 

 tion rather than by close adaptation to the environment. 



In speculating on the origin of species we may con- 

 ceive it possible, or rather certain, that among the 

 innumerable variations which occurred among the vast 

 multitudes of low unicellular organisms, such a variation 

 occasionally occurred as the following : that when one 

 cell divided into two the resulting cells did not separate, 

 as normally happened, but remained adherent; and 

 further, that this variation, whether for purposes of 

 food-getting, locomotion, protection, &c., proved a fortu- 

 nate one. This variation, which, like other variations, 

 would tend to be transmitted, and which, if fortunate, 

 would tend to cause the ultimate survival of those 

 organisms that possessed it, would be the first step in 

 the evolution of the multicellular from the unicellular 

 organism. The dual animal which resulted would re- 

 produce by each of its cells dividing into two, so that 

 there would be four single cells which would separate, 



