What knowledge did Darwin’s theory need in order to be tested, which he didn’t have at the time?

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.

1. is geographic distribution

2 is increasing complexity

3 is transitional forms

answerwestern plateau,     central lowlands,     great dividing range;