As the amount of modification which animals of all kinds undergo partly depends on the lapse of time, and as the islands which are separated from each other, or from the mainland, by shallow channels, are more likely to have been continuously united within a recent period than the islands separated by deeper channels, we can understand how it is that a relation exists between the depth of the sea separating two mammalian faunas, and the degree of their affinity, a relation which is quite inexplicable on the theory of independent acts of creation.
espezie gutxi baino ez izatea forma endemikoen proportzio handia dutenak; zenbait taldetako kideek aldakuntzak izatea, baina ez horrela klase bereko beste talde batzuetakoak; ordena osoak falta izatea, batrakioak eta lehorreko ugaztunak kasu, nahiz eta saguzar hegalariak bai izan;