finally they become play, though here also it is probable that 

use is not excluded. Thus, so far as the male bird is concerned, 

bird-song possesses a primary prenuptial significance in 

attracting the female, a secondary nuptial significance in 

producing excitement (p. 48). He holds also that the 

less-developed voices of the females aid in attaining the same 

end (p. 51). Finally, bird-song possesses a tertiary extranuptial 

significance (including exercise play, expression of gladness). 

Haecker points out, at the same time, that the maintenance of some 

degree of sexual excitement beyond pairing time may be of value 

for the preservation of the species, in case of disturbance 

during breeding and consequent necessity for commencing breeding 

over again. 

Such a theory as this fairly coincides with the views brought 

forward in the preceding pages,--views which are believed to be 

in harmony with the general trend of thought today,--since it 

emphasizes the importance of tumescence and all that favors 

tumescence in the sexual process. The so-called esthetic element 

in sexual selection is only indirectly of importance. The male's 

beauty is really a symbol of his force. 

It will be seen that this attitude toward the facts of tumescence 

among birds and other animals includes the recognition of dances, 

songs, etc., as expressions of "gladness." As such they are 

closely comparable to the art manifestations among human races. 

Here, as Weismann in his _Gedanken ueber Musik_ has remarked, we 

may regard the artistic faculty as a by-product: "This [musical] 

faculty is, as it were, the mental hand with which we play on our 

own emotional nature, a hand not shaped for this purpose, not due 

to the necessity for the enjoyment of music, but owing its origin 

to entirely different requirements." 

The psychological significance of these facts has been carefully studied 

and admirably developed by Groos in his classic works on the play instinct 

in animals and in men.[27] Going beyond Wallace, Groos denies _conscious_ 

sexual selection, but, as he points out, this by no means involves the 

denial of unconscious selection in the sense that "the female is most 

easily won by the male who most strongly excites her sexual instincts." 

Groos further quotes a pregnant generalization of Ziegler: "In all animals 

a high degree of excitement of the nervous system is _necessary to 

procreation_, and thus we find an excited prelude to procreation widely 

spread."[28] Such a stage, indeed, as Groos points out, is usually 

necessary before any markedly passionate discharge of motor energy, as may 

be observed in angry dogs and the Homeric heroes. While, however, in other 

motor explosions the prelude may be reduced to a minimum, in courtship it 

is found in a highly marked degree. The primary object of courtship, Groos 

insists, is to produce sexual excitement. 

It is true that Groos's main propositions were by no means novel. Thus, as 

I have pointed out, he was at most points anticipated by Tillier. But 

Groos developed the argument in so masterly a manner, and with so many 

wide-ranging illustrations, that he has carried conviction where the mere 

insight of others had passed unperceived. Since Darwin wrote the _Descent 

of Man_ the chief step in the development of the theory of sexual 

selection has been taken by Groos, who has at the same time made it clear 

that sexual selection is largely a special case of natural selection.[29] 

The conjunction of the sexes is seen to be an end only to be obtained with 

much struggle; the difficulty of achieving sexual erethism in both sexes, 

the difficulty of so stimulating such erethism in the female that her 

instinctive coyness is overcome, these difficulties the best and most 

vigorous males,[30] those most adapted in other respects to carry on the