A SATUEATED ATMOSPHERE. 186 



the commencement of the wet season a multitude of young 

 larvae are to be found, which could not be the case if these 

 old individuals had not till then coupled and laid eggs. Thus 

 the eggs, though minute and only enclosed in a thin integument, 

 show an especial resistance to drought. A parallel case is 

 that of the eggs of many aquatic creatures which exhibit a 

 power of enduring drought. I refer the reader on this point 

 to what has been said above. But that the eggs of insects 

 laid in the air, although perfectly protected by their envelopes, 

 are not wholly impermeable to the air — that, on the contrary, 

 they require that it should find access to the protoplasm of 

 the ovum-cell — is proved by the following easily conducted 

 experiment. If the eggs of insects are covered with a very 

 thin film of resin or of oil, which prevents the passage of any 

 air whatever through the pores of the integument, the embryos 

 perish without exception, since the oxygen requisite for their 

 respiration cannot penetrate to the protoplasm. Hence it 

 follows that even though the ovtim-cell may be partly pro- 

 tected against desiccation by the envelope surrounding it, yet 

 the perfect immunity shown by the eggs of most insects must 

 be partly due to the properties of living protoplasm. 



II. The influence of a saturated atmosphere. — In many 

 cases the moisture of the air reaches the maximum attainable 

 under the existing temperature. This is not unfrequently the 

 case, for instance, in European countries in the winter, and in 

 the tropics during the rainy season, or under the leafy shade and 

 protection of the primeeval forests. 



Unfortunately we know next to nothing as to the influence 

 of such absolutely damp air on the animals living in it ; we 

 can only say that it is highly injurious to some, and to others 

 again particularly advantageous. An extremely remarkable 

 fact, depending on this, in the geographical distribution and 

 habits of life of certain animals needs a closer discussion. 



We should at first sight be naturally disposed to assume 

 that species of animals whose organisation indicates adapta- 

 tion to breathing water and to moving in water would be 

 incapable of living in the aii', since their skin must soon diy up 

 in the air, so that it could no longer fulfil the functions proper 



