All interpretations made by a scientist are hypotheses, and all hypotheses are tentative. They must forever be tested and they must be revised if found to be unsatisfactory. Hence, a change of mind in a scientist, and particularly in a great scientist, is not only not a sign of weakness but rather evidence for continuing attention to the respective problem and an ability to test the hypothesis again and again.

It seems to me that for Darwin the pulsing of evolutionary rates was a strictly vertical phenomenon.

Two forms or species are sympatric, if they occur together, that is if their areas of distribution overlap or coincide. Two forms (or species) are allapatric, if they do not occur together, that is if they exclude each other geographically. The term allopatric is primarily useful in denoting geographic representatives.

Isolating mechanisms are biological properties of individuals that prevent the interbreeding of populations that are actually or potentially sympatric.

According to the concept of transformational evolution, first clearly articulated by Lamarck, evolution consists of the gradual transformation of organisms from one condition of existence to another.

Given the fact of evolution, one would expect the fossils to document a gradual steady change from ancestral forms to the descendants. But this is not what the paleontologist finds. Instead, he or she finds gaps in just about every phyletic series.

Evolution ... is opportunistic, hence unpredictable.