Page:George Archdall Reid 1896 The present evolution of man.djvu/49

organisms only conjugate at rare intervals? the answer may be given in the following extract—

 

"It has often been alleged that the subsequent dividing 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 conjugation, a single individual had, by ordinary asexual division, given rise to a family of from forty to fifty thousand 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. Conjugation 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 multiplication rather than by close adaptation to the environment.

In speculating on the origin of species we may conceive 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 fortunate 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 reproduce by each of its cells dividing into two, so that there would be four single cells which would separate,