The subject of rhythmic recurrence is worthy of further study. This recurrence in combination with evolutionary change may account for the apparent marked difference between Cambrian and Precambrian times, a difference which strikes some geologists as being too great to be accounted for as due to our ignorance only. _Organic evolution._ This subject is too wide for more than passing notice in a work of this character. The evidence of Palæontology is of extreme importance to the biologist, and indeed, the way in which evolution of organisms has occurred can only be actually demonstrated by reference to Palæontology, and the study of Palæontology has already given much information concerning the lines on which evolution has proceeded in different groups of organisms. It must be remembered that the major divisions of the invertebrata were in existence in very early times; indeed representatives of most of them are found in the rocks containing the earliest known fauna, that of the _Olenellus_ beds of Cambrian age. If our present views as to evolution be correct, there is no doubt that the period which elapsed between the appearance of life upon the globe and the existence of the _Olenellus_ fauna must have been very great, possibly, as Huxley suggested, much greater than that which has elapsed between early Cambrian times and the present day. If this be so, however probable it is that we shall carry our knowledge of ancient faunas far back beyond Cambrian times, it is extremely improbable that we shall ever get traces of the very earliest faunas which occupied our earth. _Geological time._ Various attempts have been made to give numerical estimates of the lapse of time which occurred since the earth was formed, or since the earliest known rocks were deposited. These attempts may be classed under two heads, namely, those made by physicists, mainly on evidence obtained otherwise than by a study of the rocks, and those made by geologists by calculating the mean rate of denudation and deposition of the rocks, and estimating the average thickness of the rocks of the geological column. The calculations of physicists as to the age of the earth vary:--Lord Kelvin assigned 20,000,000 years as the minimum and 100,000,000 as the maximum duration of geological time. Prof. Tait has halved Lord Kelvin's minimum period, while Prof. G. Darwin admits the possibility of the lapse of 500,000,000 years.