208 THE EVOLUTION OF SEX. 



V. The Antithesis between Growth and Multiplication, 

 between Nutrition and Reproduction. — The life of organisms is 

 conspicuously rhythmic. Plants have their long period of vegetative 

 growth, and then suddenly burst into flower. Animals in their young 

 stages grow rapidly, and as the growth ceases reproduction normally 

 begins. Or again, just as perennial plants are strictly vegetative 

 throughout a great part of the year, but have their stated recurrence 

 of flowers and fruit, so many animals for prolonged periods are 

 virtually asexual, but exhibit periodic returns of a reproductive or 

 sexual tide. In some cases, such as salmon and frog, periods of 

 active and preponderant nutrition are followed by times of fasting, at 

 the end of which reproduction occurs. Foliage and fruiting, periods 

 of nutrition and crises of reproduction, hunger and love, must be 

 interpreted as life-tides, which will be seen to be but special expressions 

 of the fundamental organic rhythm between sleep and waking, rest 

 and work, upbuilding and expenditure, which are expressed on the 

 protoplasmic plane as anabolism and katabolism. 



The common hydra, in abundant nutritive conditions, produces 

 numerous buds, and even these sometimes begin themselves to bear 

 another generation. In other words, we may almost say, with plenty 

 of food the polype grows abundantly, so obviously is this asexual 

 reproduction continuous with growth. A check to the nutritive condi- 

 tions, however, brings on the development of the sexual organs and 

 the occurrence of sexual reproduction. In planarian worms, the 

 asexual multiplication of which we have already noted, Zacharias 

 observed that favorable nutritive conditions were associated with the 

 formation of asexual chains, while a check to the nutrition brought 

 about both the separation and the sexual maturity of the links. 

 Rywosch corroborates this, noting in Microstomum /ineare that the 

 generative organs do not become completely matured till the indi- 

 viduals cease to be links in a chain, and that the sexuality is hastened 

 by outside influences such as checked nutrition. The gardener 

 root-prunes his apple-tree, thereby checking nutrition to improve the 

 yield of fruit, in other words, to augment reproduction. Reversely, 

 the removal of reproductive organs may increase the development of 

 the general "body" both in plant and animal, — witness the castrated 

 ox, capon, &c, or the way in which the gardener nips off the flower- 

 buds from his foliage plants. Taking a further step, we recall the 

 familiar and already repeated fact that favorable nutritive and other 

 conditions enable the aphides to continue parthenogenetic through the 

 summer months; but both for the common plant-lice and for the vine- 

 insect phylloxera, it has been shown that a check to nutrition causes 

 the parthenogenesis to cease, and is associated with the return of 



