Again, we are not in a position to form any opinion as to the causes which really influence the pairing of animals when choice is exercised. We have no certain knowledge upon the important question whether the ideal of beauty, if possessed by the lower animals at all, is in all, or even in many cases, in accordance with our own. We, for instance, admire a male humming-bird; what certainty have we that he is equally beautiful in the eyes of his mate? In cases where we have reason to believe that deliberate selection has taken place, we do not know that that selection was influenced by only one condition--that of beauty. There may have been a thousand causes at work of which we know nothing. Mr. Darwin brings forward an instance in which the owner of a number of peahens wished them to breed with a peacock of a particular variety, while they showed a deliberate preference for another bird; and he supposes that their preference was decided by the plumage. But there might have been another cause--at least the circumstances as related by him seem to suggest it--which would give a very different turn to the affair. The favoured peacock, spoken of as "old," [Footnote: Descent of Man, vol. ii. p. 119.] was probably an old friend of the hens, while his unsuccessful rival seems to have been a new introduction. The preference shown by the hens would in this case be fully accounted for, without supposing them to have exhibited any choice in the matter of plumage. Then there are a vast number of peculiarities which are certainly not ornamental in our eyes, but which are confined to the male sex. They are, so far as we can tell, of no service whatever in the struggle of life. With reference to these Mr. Darwin's argument seems to be this,--"They can serve no other purpose with which we are acquainted, therefore they must be attractive to the female--therefore they must be acquired by sexual selection." Such arguments as these cannot carry much weight. [Footnote: Descent of Man, vol ii p 284.] On the whole, we can hardly come to any other conclusion than that the theory of sexual selection is not proved. In many cases it is known that such selection is not the result of choice; in other cases, where choice seems probable, we have no ground for believing that external appearance is the sole ground of that choice. It may exercise some influence, but that is all. Even if admitted, there are many things which cannot be accounted for by it without very extravagant assumptions. It cannot then be admitted as covering the large classes of phenomena left unaccounted for by the theory of natural selection. So far as the lower animals are concerned, the results to which an examination of Mr. Darwin's views has led us may be summed up in the following propositions:--