"発達" means getting closer to a more completed state as it grows. We use "発達" about the functions and natural phenomena of living organisms and organs, such as ’発達 of mind and body", "発達したdogs of olfaction" and "developed low pressure". "発展" will expand as the momentum and power of things increases. is this enough for you?

発達：生物や身体の機能が成長すること。 発展：物事の勢いが増し広がっていくこと。 Both in English, development and they are close meanings. Bat. We use '発達' for living things and the body.