76 THE IXFLFENCE OF INANIMATE SURIiODNDINGS. 



we find true chlorophyll in animal tissues, to recognise in its 

 presence a singular and interesting case either of parasitism or 

 of the community of two organisms so difierent as an animal 

 with true tissues and organs, and a one-celled plant.'* 



The general relations between light and the vital activity 

 of animals. — By far the larger number of animals are conscious 

 of light by means of the eye only. This was directly proved 

 by the interesting experiments of Lister and Pouchet, which will 

 be more fully described further on. The commonest effects of 

 light, of its different degrees of intensity, and of its total absence 

 are familiar to all. They are exhibited every day in regular 

 succession in ever}' animal that lives ; darkness induces sleep in 

 diurnal animals, and with this are connected certain other 

 effects on some of the organs and their functions ; for instance, 

 the amount of carbonic acid exhaled by the Mammalia during 

 sleep is different from that exhaled when they are awake. 

 These proportions, however, are of no particular intereist in this 

 place. What is far more important is the observed and well- 

 .iscertained fact that all active diurnal creatures fall asleep 

 promptly during an ec'.ipse of the sun ; the darkness deceives 

 them as to the hour, and so interrupts the periodicity of their 

 vital activity. But all animals do not react in the same way 

 under the alternation of light and darkness; while some — the 

 diurnal animals — go to rest at the approach of night, others, 

 nocturnal animals, then rouse up, and we might be tempted by 

 this to divide them into day and night animals. But such 

 a division has merely a biological value, for we know that 

 it is in no way co-extensive with the conditions of affinity in 

 animals. We are acquainted with diurnal and nocturnal species 

 among the Mammalia as well as among Birds; some Butterflies, 

 Beetles, and other Insects are nocturnal, though the gi-eater 

 numbfr fly by day ; nay, even within the limits of quite small 

 families or even genera, there are some species which are lively 

 by day and others by night. To give one example only : every 

 entomologist knows that night-flying Lepidoptera, nocturnal 

 as to their affinities and structure, as, for instance, the iSesice or 

 Aglia Tau and others, rest by night and fly gaily about by day 

 to seek food or to seize the female. The causes of these 



