How has internal and external fertalization been successful in relation to the colonisation of terrestrial and aquatic environments?So, how hsve...

External fertilization occurs only in aquatic species;
males release their sperm, and females their eggs, directly into the water, and the
sperm then have to find their way to the eggs. This is a very successful fertilization
method for many freshwater and ocean dwelling species, particularly for those that are
sessile (remaining in one location) like corals. However, these species are permanently
limited to living in a water environment.

Biologists
believe that the development of the amniotic egg, which kept the embryo from drying out
without needing to be kept submersed in water, was a key turning point that allowed
animal species to colonize the land. However the waterproof covering of the egg would
also present a significant barrier to sperm, so these eggs need to fertilized before the
covering is completed. This necessitated the development of internal fertilization. In
some more primitive species the transfer of sperm into the female reproductive tract is
fairly simple and direct, while in more evolved species this has become quite complex,
leading to highly developed copulatory organs and mating
behaviors.

Ultimately, without the development of internal
fertilization, the only land dwelling crreatures we would have would have a life history
similar to that of amphibians, which are dependent on water for their reproductive
cycles to be completed.