The most common cause is the accuracy of the measure instruments. And the observation of operator is also an important factor. That's why one measurement should do three times and use the average.
The fish has a single circulation blood passes through the one in each circuit. the fish's heart has two chambers. the first one is the atrium and the second one is the ventricle. the blood entering the heart of the fish goes to the atrium first and then to the ventricle. the ventricle then pumps the blood to the gills.
a population can become more adapted to its environment through natural selection. natural selection is a process by which organisms that are well adapted to their environment survives and produce more offspring. The least adapted organism die and are unable to give rise to new generation with characteristics that are not well adapted to the environment. the process is also referred to as survival for the fittest. this is the main process that brings about evolution. the theory of natural selection was fully expounded by scientist Charles Darwin.