2 24 THE EVOLUTION OF SEX. 



have shown us that many marine algos reproduce during the 

 darkness of the Arctic winter. 



What is true for the cell, is true .for cell-aggregates. 

 Organisms in their entirety have very definite limits of growth. 

 Increase beyond that takes place at a risk, hence giant varia- 

 tions are peculiarly unstable and short-lived. Or again, just as 

 the single cell has found, probably somewhat pathologically, a' 

 surface-gaining expedient in the emission of mobile processes, 

 so many organs, notably leaves, have struck a balance between 

 mass and surface by becoming split up into lobes and more or 

 less discontinuous expansions. 



Spencer has laid great stress on the importance of the 

 physiological capital with which the organism begins ; this 

 represents, in active animals at least, the start which their 

 anabolism gets at the outset. Other things equal, growth varies 

 — {a) directly as nutrition ; {!>) directly as the surplus of nutri- 

 tion over expenditure ; (c) directly as the rate at which this 

 surplus increases or decreases ; {d) directly (in organisms of 

 large expenditure) as the initial bulk ; and {e) directly as the 

 degree of organisation,— the whole series of variables being 

 finally in close relation to the doctrines of the persistence of 

 matter and conservation of energy. Some apparent exceptions 

 are readily explained. Thus, many plants seem to grow in- 

 definitely, but they expend very little energy, and have often 

 enormous surface area in proportion to mass. The crocodile 

 goes on slowly growing, though at a gradually diminishing rate, 

 but it again expends relatively little energy in proportion to its 

 high nutrition. Birds which expend most energy, have their 

 size most sharply defined. 



§ 5. The A?itit/iesis 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. Ani- 

 mals 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 



