1 88 The Study of Animal Life part in 



and order, species and species, an antithesis may be read 

 between predominant activity and preponderant passivity, 

 between lavish expenditure of energy and a habit of stormg, 

 between a relatively more disruptive (katabolic) and a re- 

 latively more constructive (anabolic) series of changes in 

 the protoplasmic life of the creature. The contrast between 

 the sexes is an expression of this fundamental alternative of 

 variation. 



The theory is confirmed by contrasting the characteristic 

 product of female life — passive ova, with the characteristic 

 product of male life — active spermatozoa ; or by summing 

 up the complex conditions (abundant food, favourable 

 temperature, and the like) which favour the production of 

 female offspring, with the opposite conditions which favour 

 maleness ; or by contrasting the secondary sexual char- 

 acters of the more active males (e.g. bright colours, smaller 

 size) with the opposite characteristics of their more passive 

 mates. 



Apart from the general problem of the evolution of sex, 

 those who find the subject interesting should think about 

 the evolution of the so-called " sexual instincts," as illus- 

 trated in the attraction of mate to mate. As to the actual 

 facts of pairing and giving birth, it seems to me that I have 

 suggested the most profitable way of considering these in a 

 former part of this book where courtship and parental care 

 are discussed, though I believe firmly with Thoreau, that 

 " for him to whom sex is impure, there are no flowers in 

 nature." 



2. Divergent Modes of Reproduction. — (a) Herma- 

 phroditism. — Especially among lower animals, both ova 

 and spermatozoa may be produced by one individual, 

 which is then said to be hermaphrodite. So most common 

 plants produce both seeds and pollen. Some sponges and 

 stinging animals, many " worms," e.g. earthworm and leech, 

 barnacles and acorn-shells among crustaceans, one of the 

 edible oysters, the snail, and many other molluscs, the sea- 

 squirts, and the hagfish, are all hermaphrodite. But it 

 should be noted that the organs in which ova and sperma- 

 tozoa are produced are in most cases separate, that the two 



